The book and movie obviously overlap quite a bit. But the book certainly fills in many gaps that the theatrical releases deleted. That's why I enjoy the Saga when it's on tv, to answer some of the open questions. I wish all versions of the film had the scene when Vito & his sons visit Genco during his last hour. I think it would have given GF1 more support of Michael and Vito's strained relationship. I don't think "That's my family Kay, it's not me" does it justice. I also liked the fact that Santino's character was more fully developed in the book (as expected due to time constraints in a movie). I could have done without the detailed description about Sonny's genitalia! But probably the best portion from the book is the exchange between Tom & Vito after the sitdown. To my knowledge, this doesn't appear in any film. And, not to be overlooked, the whole Woltz thing in the book was so cool, especially after he called Tom in a rage after Khartoum was decapitated.
The end of the book with Tom's explanation to Kay was unexpected. But, I don't believe that the book's ending had a proper place in the movie.
Anway, I plan on re-reading the book soon. It was neat to see the movie first and then read the novel.