Hmmm... OK, I've got two.
Zen and the Art of Motorcycle Maintenance by Robert M Prisig. It's very difficult to describe what this book is about... It's basically a long, rambling exploration of just why it should be that logic and reason seem so blind to a quesion Plato posed as: "What is good, Phaedrus, and what is not good? Do we need anyone to tell us these things?" Some questionable metaphysics, but definitely a life-changer for me. Apparently he was clinically insane when he wrote it...
Overcoming Depression by Paul Gilbert. It kinda irks me to mention a self-help book, but this (and the rest in the series) are somewhat atypical. Rather than the usual "feel-good" twaddle, it's a rock-solid handbook to self-administered Cognitive Behavioural Therapy. It was (and still is, sometimes) hard work to put it into practice, but it certainly helped me enormously and I can't recommend it enough to anyone either suffering from depression themselves or who knows someone who is.