The 9th Configuration by Peter Blatty. Thought-provoking...
and maybe even life-changing. (Does hope exist in a world that is constantly trying to destroy itself?)
"This is a well written and tightly crafted novel. The dialogue, while often full of angst, is witty and clever. The premise of the book is certainly unusual. A secret facility houses militairy officers, all brave men, who have, for reasons unknown, become mentally disturbed. Housed without hope, a militairy psychiatrist enters their lives in a most unexpected way. Yet, it appears that he has his own issues with which to contend, issues that lie at the heart of his seeming understanding of their problems. Things are not always what they seem. Be prepare for a shocking surprise. This book is definitely a thriller in every sense of the word." Read more reviews here.
Psst. The film is excellent too.