I saw the movie based on this book three years back, and I (now) can attest it was a remarkably faithful adaptation.
The book is set in the 1950s. Teddy Daniels is a U.S. Marshal, off with his new partner, Chuck Aule, to Shutter Island, a (fictional) Boston Harbor Island. Shutter houses a prison/hospital for the criminally insane, and one patient, Rachel Soldano, has gone missing. Out of a secure facility, without anyone noticing. How did she do that?
But things are not what they seem. For one thing, Teddy is a widower, his wife having died in a fire set by one Andrew Laeddis. Andrew is also a prisoner/inmate at Shutter Island, and Teddy would like to kill him, in order to resolve his own inner mental turmoil. But Laeddis is as hard to find as Soldano. Also, there's a very bad hurricane bearing down.
Are the inmates actually running the asylum? It seems that might be the case. Shutter is filled with creepiness, danger, colorful insanity, and amateur cryptography. It all is based on some very vile and shocking behavior.
Dennis Lehane is a fine writer; I got hooked on his Kenzie/Gennaro private eye series years back. Shutter Island might be a tad overwritten, but is a compelling page-turner. I don't know if you should take the effort to read it if you've seen the movie, though.