Since now I've read both the book, and seen the movie, it's time for my take on which one is better the book or the movie.
This one is definitely a tough one, because there are plusses to both. If you read the book first, you will be completely satisfied, it is a page-turner, and you will not be disappointed. I found it quick to read because I became obsessed with seeing what Lehane had in store for me next.
Usually, once I've seen a movie that I didn't realize was a book in advance, there's always someone who says to me, "you should read the book, it's so much better than the movie". I don't think that this was the case with Shutter Island. To me, even though the book gave much more in way of description, I did not find that there was a clear winner this time between the novel and the film.
There's no question about it, the movie was well made, and fit the book very well. As far as deciding on this one, unfortunately I'm going to have to say if you've read the book, don't see the movie right after you've read it. Wait until the story has faded in your mind a bit. If you see the movie, unless you love the plot, you don't necessarily need to read the book. But if you have to choose just one, I'd say go with the book. The details in the book put the puzzle together a lot easier for audience, and I think it leaves you a lot more satisfied with the outcome of the story.