Gaea trilogy

The Gaea Trilogy consists of three science fiction novels by John Varley. The stories tell of humanity's encounter with a living being in the shape of a 1,300 km diameter Stanford torus, inhabited by many different species, most notably the centaur-like Titanides, in orbit around the planet Saturn.

The novels are:
 * Titan (1979)
 * Nebula Award nominee, 1979; Hugo Award nominee, Locus Award winner, 1980
 * Wizard (1980)
 * Hugo and Locus Award nominees, 1981
 * Demon (1984)
 * Locus Award nominee, 1985