I hate trilogies. I really do. Most of the time, the author barely writes anything interesting in the first book, let alone those that follow.
Having said that, let me tell you that John Varley has written one of the great SF stories of the last twenty-five years. "Titan" (with its companion volumes, "Wizard" and "Demon") tells the story of Cirocco Jones and the crew of the NASA spacecraft DSV Ringmaster as they meet, are changed by and ultimately must fight Gaea, the living planet. The characters are vivid and well-drawn; Varley has justifiably drawn praise for his strong, realistic female characters.
Like Jack Chalker did in "Well of Souls", Varley gives us a powerful story of humanity lost and regained in the face of epic struggles. His reach sometimes exceeds his grasp -- especially in "Demon" -- but I recommend this trilogy wholeheartedly.