The Ringworld Engineers

Louis Wu, the wirehead veteran of the first Ringworld expedition, is abducted by the Hindmost, the deposed leader of the Puppeteers, along with the Kzin known as Chmeee. The Hindmost wants the technology of the Ringworld to win back his power, and he sends the pair down to explore the artificial world while he waits aboard his ship.

Louis and Chmeee soon plan to overthrow their captor so they can go home, but the Ringworld has problems of its own. Across its enormous surface they meet several hominid species that evolved there, and they learn that the structure is unstable and will eventually drift into its own sun. Saving it will take a desperate plan involving the world's long-silent control systems, and some of the people they meet along the way will pay for it.

The second book of the Ringworld series and the first sequel to Niven's 1970 novel Ringworld, this is hard science fiction in the Known Space universe, driven by engineering puzzles and the sheer scale of its setting. It was nominated for the Hugo and Locus awards in 1981.