Gone Baby Gone
It sounds like the movie's pretty good, but (as David Edelstein suggests) you should really, really read the book first. Not just because it's probably the best mystery novel I've read since P.D. James' A Taste For Death, but because there's no way the big plot twist (not whodunit - I had that pegged pretty early in the book - but why) and the sense of being "ripped in half," as Edelstein puts it, that comes with it, will carry the same force on screen as it does on the page. Read it first, then see what the Affleck brothers have made of the story. You won't be sorry.