The Ringworld Engineers

Over twenty years after returning to Earth from the Ringworld, Louis Wu is kidnapped, along with a warrior kzin named Chmeee, by a Pierson's puppeteer, a brilliant 2-headed alien who calls himself the "Hindmost". The puppeteer hopes to regain status with his fellow citizens by traveling to the Ringworld to bring back treasures. Upon reaching the vast and mysterious Ringworld, Louis and his companions find many surprises, including that the Ringworld is unstable. Its billions of inhabitants ...