I saw this book a while back. I really enjoyed it. I'm not sure why I didn't read the book first. That's not like me. But, even having seen the movie, I really enjoyed reading this book. I enjoyed the additional storyline. I enjoyed the storytelling. And to be honest, if the movie had followed the story a little more, I don't think they would have needed the R-rating.
Even if you saw the movie, read the book. You'll be happy you did.