The best of the Niven-Pournelle collaborations, and one of the best first-contact novels ever written.
It is set in a future human-only space empire where outlying colonies are rebelling against the central authority. An alien probe with a light-sail comes careening into one of the human planetary systems, New Scotland. The leads to an expedition to the system the probe came from, and to an encounter with the species in that system. A classic problem arises: are the aliens a terrible danger or a unique opportunity?
Niven and Pournelle do a first-class job of making the aliens (Moties) believeable. The difficult balance here is always whether to make the aliens are really alien (and thus difficult to understand) or more human-like, so the reader can relate to them well. Most authors lean towards the latter, understandably. It makes for more sympathetic, identifiable characters. You usually end up with aliens with human personalities that are a bit neurotic in some way. Like Klingons or Romulans, which are more or less bellicose humans. The Moties are like that, but with pretty well-thought-out alien motivations that drive them certain ways that mark them as not quite human. I could say more, but I'm trying to avoid spoilers.
The science is top-notch, especially the physics. I can't vouch for the biology as much, but it worked for me. And it comes to a good, logical conclusion, which I always find important. (Note that it is not a final solution, as there is a sequel) And it stands up well to re-reading, so it gets style points, too.