I saw the movie based on this novel a few years ago and then started watching it again recently and remembered the main idea of the twist but not the details. So I stopped watching it and decided to read the book instead because I loved Mystic River, also written by Dennis Lehane. Shutter Island is a great story but, unusually, the movie is a really good treatment of the book, it was very faithful to the story. If you’ve see the movie the book doesn’t really add much.
Amazon book description:
“The year is 1954. U.S. Marshal Teddy Daniels and his new partner, Chuck Aule, have come to Shutter Island, home of Ashecliffe Hospital for the Criminally Insane, to investigate the disappearance of a patient. Multiple murderess Rachel Solando is loose somewhere on this remote and barren island, despite having been kept in a locked cell under constant surveillance. As a killer hurricane relentlessly bears down on them, a strange case takes on even darker, more sinister shades—with hints of radical experimentation, horrifying surgeries, and lethal countermoves made in the cause of a covert shadow war. No one is going to escape Shutter Island unscathed, because nothing at Ashecliffe Hospital is what it seems.”