An excellent book, topped only by Protector in my opinion.
It takes place in Niven's Known Space universe. If you are not familiar with that, suffice it to say that it is a future civilization populated by humans and a variety of interesting, if somewhat anthropomorphic aliens. One such race discovers an amusing artifact, and recruits a party of humans (and one big, nasty alien) to explore it.
The artifact in question is a gigantic flat ring orbiting a star. This notion is a derivative of the idea of a Dyson sphere. Years ago, Freeman Dyson proposed that a sufficiently advanced civilization might build a sphere around its parent star. This would give them lots of living room, plus they could capture, and possibly utilize all of the energy output from the star. You wouldn't need that much material, even if the sphere's radius is about an astronomical unit (the earth-sun distance, about 93,000,000 miles). The idea of a ringworld is that you can get many of the benefits by just building a piece of that sphere, i.e., the equatorial part. If you construct it right, it can hold an atmosphere and you can spin it for gravity. And you obviously need less material. I don't know if the idea itself was original with Larry Niven or not, but this book sure publicized it.
At any rate, Louis Wu and his motley crew spend the book exploring this huge artifact. They are circumspect, since the builders of such a monstrous thing are likely to be advanced and powerful. This turns out not to be the case. Nonetheless, they are forced to crash-land on the Ringworld, and spend the rest of the time trying to survive and figure out how to get back to "civilization".
Along the way, much more is learned about the characters themselves and Known Space than is learned about the Ringworld or its inhabitants. Which is fine. We learn interesting things about breeding for luck, and why humans obtained hyperdrive from the Outsiders before the Kzinti did.
You may wonder, as I did when I first read this, whether a ring structure such as this is gravitationally stable. That is, if it is perturbed by, say, an impacting asteroid, will it maintain its position relative to its sun? An important question for the inhabitants! Being a good little physics major, I quickly determined that the answer is no, it is unstable against such perturbations. The calculations involve Bessel functions, but it is clearly non-stable. So, why is it still there? What provides the stability? Also, others noted that all of the topsoil on the ring should have washed into the lakes and oceans hundreds of thousands of years prior to Louis's arrival. These and other questions, including the penultimate one of who the Builders were, are not answered here. Yet the story is so good, one is less frustrated than one might think. And after about 10 years of nagging (and "helpful" suggestions), Niven gave in and wrote a sequel, explaining most of these points. It's called The Ringworld Engineers and I don't have a review done for it yet.