The Ringworld Engineers: The Ringworld Series, Book 2

It's been 20 years since the quixotic and worldsweary Louis Wu discovered the Ringworld. Now he and SpeakertoAnimals are going back, captives of the Hindmost, a deposed puppeteer leader.

With Louis' help, the Hindmost intends to regain his status by bringing back such extraordinary treasures from the Ringworld that his fellow puppeteers will have to be impressed. But when they arrive, Louis discovers that the Ringworld is no longer stable-and will destroy itself within months. To survive, he must locate the control center of the legendary engineers who built the planet.

His quest becomes a wild and gripping venture blended with the mysteries and spectacular technologies that only Larry Niven can conjure.