K-PAX is one of the great underrated science fiction novels. It's a simple tale, beautifully told, which draws you in and makes you want to believe in the characters. Until the end, you could almost believe it's a true story.
Written as a first person account by a psychiatrist treating a patient who claims to be from another planet. "prot", as he calls himself, explains that he will return to his home planet of K-PAX at a specified time and date, riding on a beam of light -- and taking one of the other inmates of the hospital with him. Meanwhile, Dr. Brewer tries to understand who prot really is, and to demonstrate to himself and to prot that his patient is an ordinary human.
Too often this kind of book begins promisingly but fails to deliver on the promise -- the mysteries set out before the reader are explained away too easily, or the delicate sense of almost-reality that these stories depend on is broken. But K-PAX delivers on all fronts. Every explanation has another mystery hidden inside it, and the ending is at once realistic and rational, making sense of everything so far, while leaving some doubt and hope in your mind.
When I read K-PAX, I immediately went out and bought another copy for a friend. He phoned me up later: "I got home at midnight and saw you'd left a book for me. I thought, 'That's nice, I'll read for half an hour or so before I go to sleep." And I couldn't put it down! You &*$@*?#!". It was 4:30 in the morning.