The Gaean Trilogy (John Varley, 1979)

When Cirrocco Jones, captain of the spaceship Ringmaster, and his crew are captured by Gaea, a planet-sized creature that orbits around Saturn, they find themselves inside a bizarre world inhabited by centaurs, harpies, and constantly shifting environments.

Similar books:
Ringworld, by Larry Niven
Rendez-vous with Rama, by Arthur C. Clarke