I'm a pretty good fan of crime novelist Dennis Lehane, and I read the book version of Gone Baby Gone back in 2003. This is one of the most faithful movie adaptations of a book I can remember, from script, to atmosphere, to casting. Ben Affleck does a great job directing; he also co-wrote the screenplay.
Private eye Patrick Kenzie and his lover/partner Angie Gennaro are brought into a child abduction case that has so far stumped the Boston cops. Angie (wisely, it turns out) has misgivings, but good-hearted Patrick thinks (wrongly, it turns out) that they can't do any harm by trying. It doesn't help that the missing little girl has a foulmouthed drug-abusing slutty mother.
The movie is shot in some of Boston's least appealing neighborhoods and deals with some of its scummiest characters. It is unrelievedly grim and gritty, with nary a side trip to Fenway or the Common.
And yes, this does make the fourth Casey Affleck movie I've watched this month. Good catch.