Shutter Island by Dennis Lehane

I like to read the books before watching the movie, since the books are usually so much better. This story started out a little slow, but once you get into it you cannot put it down. Taking place on an island once used as a fort and possibly by pirates before that, it has been turned into a mental hospital. The protagonist is a marshal sent to find a missing person, who was not really missing. Once you think you know what is happening, the partner becomes a little suspicious and then disappears leaving the Marshal to determine what is really happening on this island and more importantly how to leave.