There are 3 books in the Gaia series. The newest book is Demon which came out in 1987.

Twenty years ago, the Gaean Trilogy dazzled critics and readers. Now a new generation will discover that brilliant world--beginning with Titan.
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2
Wizard

Wizard
An incredible planet-sized sentient creature orbiting Saturn, Gaea suffers from a debilitating breakdown, a form of schizophrenia that threatens Gaea's inhabitants as the planet wages war against itself

3
Demon

Book cover of Demon
The satellite-sized alien Gaea has gone completely insane. She has transformed her love of old movies into monstrous realities. She is Marilyn Monroe. She is King Kong. And now she must be destroyed.