The story is very simple, the dialog is incredibly lifelike and readable. The characters are very real and the story is believable. Its not really a mystery as much as it is a story about some criminals and what they think and feel.
Warning, though. Don't buy this if you like the 'high concept' plots of Grisham and Patterson. This is a very simple story about real people and real criminals. If you're an aspiring writer of crime fiction, definitely check it out especially the dialog.
The book has been greatly lauded for its simplicity, dialogue, and realistic characters. However, my own reading was that everyone in the book (men, women, law, criminals) spoke more or less the same clipped wise guy talk as everyone else, and not only that, but other than talking about the "Broons" (Boston's pro hockey team, the Bruins), there's little that differentiates the speech from that of countless New York and Brooklyn gangsters. So much so that one occasionally has a hard time keeping track of who is who. So, maybe it was revolutionary to reveal the inner woes of criminals back in 1970, but read today, the book lacks the punch it must once have held.