Ringworld's Children by Larry Niven
The Ringworld is dying. And this time even Louis Wu - captive of the hyper-intelligent alien Tunesmith - may be unable to save it. Even if Louis can escape he will then have to survive in a galaxy ravaged by interstellar war and deadly political intrigue. For the Ringworld is no longer a secret and entire civilisations now battle to control its power. The victorious race that conquers the Ringworld will conquer the galaxy ...and no one will be able to stop them. But Louis Wu is going to try.