Black Mass is the telling of how a corrupt FBI agent who actually grew up with Whitey, gave him carte blanche to the criminal world in return for being an "Informant". I use the term loosely, because I'm not convinced much of what Whitey gave the FBI as information actually did any good. I am amazed and equally disgusted at how much Bulger and his fellow mobster, Stephen Flemy got away with. People died because the FBI failed to inform them they were being targeted. The corruption within the Boston branch of the federal agency is astounding. 
 I do recommend reading this one. While it is slow in parts, I felt like the writers gave us a through accounting of what happened. The audio version that I listened to has an updated epilogue and notes from the authors that tells of Whitey's capture and subsequent conviction.