I did not expect this to be so good. It's not a literary masterpiece -- in fact, the writing is laugh-out-loud bad in places -- but it's got a great plot and it holds up surprisingly well compared to the movie. There are far more details in the book that add depth and background to the characters and scenes in the movie. This is a guilty pleasure kind of a read, but anyone who likes crime thrillers should enjoy this. Puzo's pacing and plotting makes up for the turgidity of his writing. There's a genius to every move in the Corleones' war, and an explanation for a lot of character actions, that's lost in the cinematic version.
I can't really explain my love for this book. It's way outside my usual genre and frankly, it's kind of schlocky, but I have to give it five stars because I enjoyed it so much. It'd definitely worth reading if you've only seen the movie.