I finally got to see this movie. It was awesome!!!! Not quite historically accurate (I grew up during the actual Winter Hill gang wars, hearing about Whitey Bulger, Bussing in Southie, and all that went with it) It was really good but I decided to make a list of the books out on the subject that tell the entire bizarre tale.
Black Mass
Rat Bastard
The Brothers Bulger Whitey Watch

I've read all of these and they are great books about the Irish Mob in Boston. Check em out if you liked the movie and get a chance. You won't be dissapointed