Below is an annotated list of a lot of the various field guides (and floras) I've used. I've included bird books, as well, for lack of better place to put them. I cannot stress how important it is to get your hands on multiple sources if you want a good chance of getting a positive identification of your plant, even if you have a supposedly authoritative flora (such as the Jepson Manual for California). The web has recently become a fantastic resource for verification of tentative identifications; I have many resources linked from my root botany page and my notes pages.
T Niehaus, "Peterson F G Pacific States Wildflowers" (1976): This was my first, and it is still one of my favorites. It is arranged by family and flower type within fairly useless broad color groups. Each page of overly-brief descriptions is faced by a page of excellent mostly black-and-white schematic drawings. It has a very useful dichotomous key in the front for determining the family. It has a (relatively) huge selection of species, but of course, no pocket guide is going to be complete. It will almost always get you to the right genus at least.
R Peterson, M McKenny, "Peterson F G to Wildflowers of NE and N-central N Amer" (1968): This is an old edition from my mom. It is nearly identical to their guide to the Pacific states (see above): It is arranged by family and flower type within fairly useless broad color groups. Each page of overly-brief descriptions is faced by a page of excellent mostly black-and-white schematic drawings. There is no key, as in the other. It has fairly good coverage (almost 40 violets alone!)
R Taylor, "Desert Wildflowers of N Amer" (1998): I love this guide. It is arranged alphabetically by family, with lots of photographs. The photos aren't always very helpful, but the descriptions are extremely good. It covers all the deserts of the American Southwest, including the Colorado Plateau. Taylor fills the text with neat scientific information about taxonomy and evolutionary adaptations to the desert. He explicitly discusses the genera and species that are tricky to identify -- most guides just give you one or two and hope you won't notice the dozens of similar species. The selection of species is pretty bad in a few places, but for the most part it is excellent. (The same author publishes "Wildflowers of the Sagebrush Steppe", which appears to be identical format.)
Pojar and MacKinnon, ed., "Plants of Coastal British Columbia" (1994):
R Spellenberg, "Nat Audubon Soc F G to Wildflowers (western region)" (2001): This is laid out the same way as their tree book (see below), with the same complaints. It is so incomplete as to be virtually useless. It covers the Great Basin and Rockies as well as the Pacific States.
L Newcomb, "Newcomb's Wildflower Guide" (1977): This covers the northeast and north central areas fairly completely (30+ species each of violets and goldenrods!) The identification key is a bit odd, based broadly on flower type, but mostly on leaf shape(!) -- my jury is still out on the usefulness of it. The descriptions are rather brief, but the facing mostly black-and-white diagrams are excellent, and he often will describe complex genera in basic detail first before diving into the multitude of similar member species.
R Smith, "Wildflowers of the Southern Mountains" (1998): This guide has a stunningly complete selection of 1200 flowers (27 goldenrods and 30 some asters alone!) covering the mountains from Maryland to the south. There is an interesting diagrammatic key that I'm not very good at using yet. Throughout, the level of technical expertise required is generally much higher than other layman's guides, but not prohibitive. The descriptions are a bit brief, and the excellent photographs are in a separate section.
E Horn, "Sierra Nevada Wildflowers" (1998): This is a nice supplement to other guides. It has lots of photos of varying degrees of usefulness. It covers the most common plants in great detail. There are a number of plants described in this book common to the High Sierras that are not covered in the Peterson guide and vice-versa. It has interesting stories and background, such as bios of the botanists that first described many of the plants in the area. It's sorted by family. It includes a brief conifer guide and key, as well as a number of shrubs, like buckeye and several ceanothus.
S Whitney, "A F G to the Grand Canyon" (1996): This is a general-purpose guide, including everything from geology and history to plants, trees, lizards, and butterflies. As expected, it only contains the most common and even misses some of the commonest species. The botany sections are sorted by flower color / leaf type, then roughly by family, with color illustrations and brief, almost useless descriptions. It is pretty good given its attempted scope, but sometimes a little frustratingly incomplete. It works well as a supplement to other guides.
S Bentley, "Native Orchids of the S Appalachian Mts" (2000): This guide covers 52 species in 21 genera in the mountains roughly south of the Potomac River. It has fantastic pictures, excellent descriptions, and detailed range maps, but no identification keys.
G Petrides, "Peterson F G Eastern Trees" (1988): This is excellent. It supplements the thorough species descriptions with lots of very handy tables comparing characteristics of similar species (such as oaks, pines, willows, etc.) It is arranged by physical characteristics, not family. (In fact family relationships are relegated entirely to a short index in the back.) All the color diagrams (not photos) of leaves, twigs, and fruits are shown in a separate section that makes a convenient identification key. There is also a leafless key in the back. Florida trees are mostly separated out in a brief appendix -- other more specific guides handle these much better. Includes leafless identification key and information about bundle scars and other highly useful but generally omitted technical twig and bud details. My only complaint is that some "miscellaneous" trees like Florida Anise and Sweetleaf are not covered in great detail.
C Brockman, "Trees of N Amer" (1968): "A Golden Field Guide". This is my mom's old guide. It covers too large an area, covering everything north of the Mexican border, inevitably missing common species here and there. The descriptions are brief and pretty unhelpful, but the range maps and facing color illustrations are very good. It is arranged by family in order of useful broad physical characteristics.
E Little, "Nat Audubon Soc F G to N Amer Trees (western region)" (1980): This seems to be extremely complete, but stays well away from shrubs -- no manzanita or ceanothus, for example. Descriptions of each species are pretty good, but frequently lack critical details about buds or twigs (the oaks are particularly incomplete, for example). All the photographs are kept in a separate section, so I find that I'm constantly flipping between them. Descriptions are arranged by families in semi-random order (at least as far as I can tell). Pictures are all photos (not diagrams), sorted by leaf shape, flower color, and seed/nut type. The "key" (such as it is) doesn't even separate alternates from opposites.
P Raven, "Native Shrubs of S California" (1966): This is pretty good. It silently omits a number of species, but it has a very good selection. It is divided up a little oddly into various groups of families, but I didn't have trouble getting used to it. Everything is keyed, right down to the species. It tells you exactly what to look for to distinguish between species. (For example, it discusses several Ephedra, manzanita, ceanothus, and lots of aster-family shrubs.) However, it rarely fully describes non-distinguishing characteristics (to use as an aid to verifying tentative identifications). There is no proper index, just a randomly sorted check list. Amazon told me it was out of print, but it wasn't hard to find a seller.
M Casebeer, "Discover California Shrubs" (2004): Lousy. This only covers 48 species. It's rather unscientific. This was the only guide on California shrubs I could find in LA book stores(!!) It is definitely not worth it in my opinion.
G Nelson, "Trees of Florida" (1994): This is excellent if you are already pretty familliar with the trees of the area. It covers all native and naturalized trees in Florida. Each species is described in brief detail, cover major characteristics, as well as helpful hints to distinguish similar species, broad distributions, many illustrations and a number of color photos. It also includes some general discussion of the families and genera in the first half of varying interest. It's a big book -- definitely not a pocket guide. I strongly recommend the Stevenson guide (below) to use as a companion: that guide will narrow down your search tremendously, then this guide will work out the tricky details.
G Stevenson, "Trees of Everglades N P and Florida Keys" (1992): This is absolutely fantastic. It's just a little pamphlet, at only 32 pages long, but it covers 130 species -- not all, but most of the native tropical trees -- with concise descriptions and illustrations. It also includes a small handful of flowers, vines, and water plants as an after thought. I believe we found this in the John Pennekamp State Park Visitor Center.
??, "Trees of South Florida" (1995): This is an 8 by 11 plasticized fold-out "cheatsheet" covering 43 trees with a color photo, and on the reverse a brief description. What it covers it does a fairly good job on, but 43 simply isn't enough to be terribly useful. You can get these, for example, at the Everglades Visitor Center.
S Grillos, "Ferns and Fern Allies of California" (1966): This is more reasonably-sized. It is very easy to use (assuming you know what an indusium is!), although it's missing some species. It has excellent thorough descriptions and fairly good diagrams of each species.
G Keator and R Heady, "Pacific Coast Fern Finder" (1981): This is a tiny thin pocket-sized book. Diagrams tell you exactly what to look for to distinguish similar species. It has dichotomous keys to get you to the right family. I might even like this better than the larger, more professional-looking "Ferns and Fern Allies of California" above. Together, the two guides give pretty good coverage. The other guide has better coverage of spike-mosses, for example, but this one has more cliff brakes.
B Cobb, "Peterson F G to the Ferns" (1963): This covers the northeast and north-central area. While ferns might not have changed too much since my mom's old edition was published, taxonomy certainly has. It has a number of useful keys of varying technicality. Each species is described in good detail with an excellent annotated black-and-white illustration. Coverage seems incomplete, but it does include horsetails, club and spike mosses, and quillworts.
G Lincoff, "Audubon Soc F G to N Amer Mushrooms" (1981): This is the only mushroom guide I've ever used, but I like it. It is arranged the same way as their other guides: each species is described exhaustively, including microscopic details about spores and gill tissue. The excellent color photographs are inconveniently placed in a completely separate section, making you flip back and forth continuously. Descriptions are arranged by "morphological" families (as opposed to the more modern phylogenetic families which are often completely unhelpful in the field). Photos are arranged broadly according to color, gill attachment, and other crude physical characteristics. There is a nominal key in the back based on spore print color. It seems to have a very good selection, including slimes, parchments, puff balls, coral fungi, etc. (But no lichens.)
Breaking news: there is a full-on dichotomous key to this guide's genera on the web at Mushroom Journal. This extremely handy key transforms this Audubon guide into a real gem.
D Sibley, "F G to Birds of Western N Amer" (2003): This is excellent. It is arranged very intelligently by order, family, and subgroup. Each species is described in detail with multiple excellent color illustrations, detailed range map, and verbal description of important songs and calls. Important identification characteristics are labeled handily in the illustrations. Coverage appears to be excellent, including many rare visitors from the east.
Robbins, et al., "Birds of North America" (1966): "A Golden Field Guide". This is my mom's old guide; it is still quite good (birds haven't evolved too much in her life time!) It covers almost 700 species, supposedly all the birds that regularly breed or visit north of the Mexican border. Descriptions are brief, but the detailed range maps and facing color illustrations are good. Sonagrams are frequently included. It is arranged intelligently by order, family, and subgroup.
Peterson, "A F G to the Birds E of the Rockies (1980): This is excellent. It covers all species in the range. Each has a brief description, including rough voice mnemonics. Illustrations on facing pages have identifying characteristics pointed out. Detailed range maps come inconveniently in a separate section at the end. It also has semi-useful silhouette "keys" of perching and flying birds at the front and back.
Martof, et al., "Amphibians and Reptiles of the Carolinas and Virginia" (1980): Each species is described and photographed in detail, including range maps down to the county. Coverage is complete, including extremely rare species. There's no key, but there aren't that many species.
Hickman, ed., "The Jepson Manual" (1996): This is the authority on California vascular plants. It is excellent, a model all other floras should emulate in my opinion. Complaints: some of the keys are a bit ambiguous, the many different authors don't always agree on precise definitions of terms, and it does not include blooming periods (yet). It is nicely complemented in southern California by the Munz flora (which I have not had much chance to use yet). There is also a desert version available with a much narrower scope. Also, there is an on-line version (part of the Jepson Flora Project) that lacks only the (excellent) diagrams and family key, but that does include (mostly) blooming times. Also, check out Tom Chester's comments; he give lots of both general advice and specific corrections.
Hitchcock and Cronquist, "Flora of the Pacific Northwest" (1974): Pretty authoritative, if a bit dated. It covers Washington, northern Oregon, northern Idaho, western Montana, and extreme southern British Columbia. Vocabulary is a bit gratuitously technical, some of their abbreviations (esp geographical) are hard to get used to, and the families are laid out according to Cronquist's out-dated classification, which is for all intents and purposes totally random. (Here is a handy family index you can print out and tape to the inside cover.) The miniature diagrams in the margins (use a hand lens!) are excellent, very convenient, and nearly complete. There are no species descriptions, making it impossible to "double-check" the keys, but the keys often use multiple characters and are particularly excellent in my experience. Cross-checking with other floras (such as Jepson) is helpful but often difficult due to extensive changes in taxonomy since 1974.
D Lellinger, "A F Manual of the Ferns and Fern-Allies of the US and Canada" (1985): This is excellent. It doesn't skimp on technical jargon, but I find it very easy to use. There are pictures of most species. It has remarkably unambiguous keys, and very thorough species descriptions, including extensive information on hybrid complexes.
I Brodo and S and S Sharnoff, "Lichens of N Amer" (2001): brief general treatment of morphology, etc, and full flora of N Amer lichens: keys, full descriptions, including many distribution maps and superb photography. Only 30% of flora covered (1004 plus 500 mentioned in passing).
I bought all these small books at the big map store in Auckland. They have several alternatives, including a few heavy-weight tomes, if I remember right. None of these are particularly expensive, but it adds up quickly.
L Metcalf, "Trees of New Zealand": This "??? of New Zealand" series is very good for new-comers to New Zealand. They are arranged by family, although I couldn't puzzle out the order of the families. Each tree has a nominal not-always-useful photo accompanied by a good long description and a very useful approximate range map. It is not complete (particularly bad about coprosmas), and sometimes it is missing pretty critical information about fruits or flowers, but it is the best of the pocket tree guides.
L Metcalf, "Ferns of New Zealand": This is just like the "Trees of New Zealand" guide. I found it very easy to use, and extremely comprehensive -- there were only two or three ferns I found that I couldn't positively identify. (Except for the Hymenophyllaceae -- those repeatedly defied my most patient scrutiny!) It includes Lycopodaceae.
G Moon, "Birds of New Zealand": Another in the same series described above. This one has no range maps. The descriptions are very thorough. I was extremely unlikely to find a bird not covered in this guide. Any difficulty in identifying species has less to do with the guide than it does the intrinsic difficulty in getting good clear viewing of the damn little beasts as they flit spasticly about the dark forest canopy.
L Metcalf, "Alpine Plants of New Zealand": This belongs to an entirely different series of guides. It is also arranged by family, but it is more difficult to use. It helps to be able to tell what family a flower is in before looking it up in this guide. (North American intuition doesn't always work, though!) This only covers about 48 main species (although some descriptions include brief descriptions of related species). It'll get you to the right genus in most cases, but you're guaranteed to be frustrated if you require definitive resolution of the species. It has good photos and long descriptions.