Finally got around to reading "The Godfather". I still hven't seen the movies. However, the book was really good. I recommend it highly.
There were many things in the book which didn't make it into the movie, as usual.
However, I'd say that the movie was a pretty fair adaption of the book.
Some things were definitely changed based on artistic license, and time constraints.
If you like Puzo's writing, I'd recommend reading "Omerta: A Novel". It was good, too.
I first saw "The Godfather" at a drive-in movie, when I was sixteen. But early on in the film, when Tom tells Sonny his dad wants to see him (and Sonny is 'hooking up' with a woman, while standing up) my girlfriend and I climbed into the back seat of my jeep, and got into some heavy 'hooking up'. So much so, we never saw the rest of the movie. So, I had to wait for it to begin appearing on television, before I actually got to see it in its entirety!