It might surprise you that I have watched the movie and read the book for this. Then again some people might be surprised that there was a book at all. But there was, written by Mario Puzo, who also contributed to the movie script.
The movie is of course excellent, consistently rated as one of the greatest films of all time. There's an extraordinary cast and a sweeping story about the corruption and rise of Michael Corleone as he takes his father's position as a mob kingpin. And in the epic finale has his henchman eliminate the heads of all the other "families" in New York to consolidate his power.
The book is OK but Puzo is not a great writer, so it reads more like a potboiler than a literary saga. The story is mostly the same. Actually a lot of the material about Vito Corleone (the Marlon Brando character) was featured in the second film so really the book is more of a compilation of Godfather Part 1 and Part 2.
I honestly don't think you need to read the book to watch the movie. It doesn't add anything you don't get if you watch the first two Godfather movies. Of course watching those movies takes like 6 hours, or almost as much as one Peter Jackson movie. Zing!