Titan: Gaean Trilogy, Book 1 (Unabridged‪)‬

John Varley's monumental trilogy - Titan, Wizard, and Demon - has achieved cult status, hailed as a modern triumph of the imagination by critics and fans.
It begins with humankind's exploration of a massive satellite orbiting Saturn. It culminates in a shocking discovery: the satellite is a giant alien being. Her name is Gaea. Her awesome interior is mind-boggling - because it is a mind. A mind that calls out to explorers...and transforms all who enter.