Might be some sort of personal record for me: I bought this book (as an SF Book Club hardcover) back in 1980 when it came out. The dust jacket has long since been destroyed. And it finally got to the top of my virtual to-be-read pile over a third of a century later. It's a sequel to Ringworld, which I reread a few months back in preparation.
Larry Niven (it is said) never intended to write a Ringworld sequel, but was inspired by fans (ka-ching!) and also by the geek-critics who pointed out that the Ringworld portrayed in the first book (a solid band with a 1 AU radius, a million miles wide, circling a Sol-class star) was seriously unstable. Any small perturbation would build, and the ring would eventually collide with its star, sooner rather than later. Yet, there it is, so there must be some sort of stabilizing mechanism that they missed describing in the first book.
This book's premise: that mechanism has stopped working, and Ringworld is seemingly doomed.
It is set a couple of decades after the events described in Ringworld. Louis Wu, the human protagonist, has returned to civilization only to become a "wirehead", addicted to a device that stimulates his brain's pleasure centers. The "Hindmost", a Puppeteer mentioned in the first book, abducts Louis and his Kzinti co-explorer (Speaker-to-Animals, now called Chmeee) for another visit.
Once they're there, things go wrong: Louis and Chmeee aren't too happy about being shanghaied and they manage to escape the Hindmost's direct control. Louis takes on the task of discovering why the Ringworld's stabilizing system went down.
Since the Ringworld's inner surface is 3 million times Earth's surface area, there's a lot to explore. They come up against a host of species which cohabit the ring in varying degrees of cooperation and strife. Most interesting is the layout of some islands in the Great Ocean: they've been engineered to be close replicas of Earth's continents and those of other known-space planets. What's going on there?
Anyway: I could understand why, back in 1980, I started reading this and lost interest a few pages in. Niven's characters aren't that interesting, or even particularly sympathetic. The setting is grand in design, but pedestrian in implementation. Reading it is kind of a trudge.