K-Pax by Gene Brewer
When a man who claims to be from outer space is brought into the Manhattan Institute, the mental ward seems to be just the place for him. However this patient is unlike anyone psychiatrist Dr. Gene Brewer has had under his care before. Calling himself 'prot', he has no traceable background but says that he is an inhabitant of the planet K-PAX, a perfect world without wars, government or religion, and where every being coexists in harmony. Setting a departure date - August 17th at 3.31am - on which he plans to return home on a beam of light, 'prot' keeps us guessing right up until the end.