Published in 1979 to tie up the many, many loose ends from his 1970 novel Ringworld (mentioned earlier here), it holds up well, even better than the first one. Might get around to the remaining two books in the series later this year, but I need a break from Niven stories for the moment.
Perhaps the most entertaining part of the book is the dedication in the front matter, where the author details all of the, um, help he had from True Fans (that last really needs the old html <blink> tag) in fleshing out the Ringworld itself. Obsessive proofreading. Design for a seafloor sediment recycling system. Calculations for the minimum tensile strength of the underlying structural material. MIT students chanting at a sci-fi convention about the inherent instability of solid rings orbiting stars. Even Freeman Dyson(!!) got in on the letter writing. In the end it was all too much for Niven, so he had to write this book. Honestly, the first one is incomplete without this, so if you read the first, be prepared to dive into the second immediately. Well, maybe sandwich Protector between reading the two, mostly for the break but also for some in-universe background.
Adventures planned, so onward with the day.