Shutter Island - Dennis Lehane

Shutter Island is a thriller with a huge twist, maybe.  The story takes place on Shutter Island, which is home to a mental institution for the criminally insane.  Two federal marshals are sent to the island to help find a missing patient.  What starts out as a locked room mystery soon expands to encompass the whole island.  I've read this novel and I'm still not sure of the ending.  I'm not sure which way I'd like it to end either.  Very highly recommended.