The Ringworld Engineers (Ringworld)

The Ringworld Engineers came out in 1979 and was nominated for the Hugo and Locus awards in 1981. It was the first sequel to Ringworld, and Niven explains in his introduction that he had never planned to write more than one.

What changed his mind was partly the demand for more and partly the arguments of his readers. Fans had gone through the engineering of the ring and found a serious flaw: a rigid structure of that kind would not in fact be in orbit around its star, so over time it would drift and eventually collide with the sun.

Niven records that a group of students at the 1971 World Science Fiction Convention chanted the problem at him, and the new novel takes it up directly. The story returns to Louis Wu, who has been living as a wirehead, and brings him back to the ring at the insistence of the Hindmost, the deposed leader of the Puppeteers, who has reasons of his own for wanting the structure examined.