I didn't remember the Godfather II flashbacks being so in-depth in the novel. Which is to say, I remember that about 1/2 (maybe less) of the flashbacks seen in the movie were present in the novel, the rest being invented (by Puzo) for the second film. I could be wrong about that, though - it's been a while since I read the book.
The "present day" stuff in Godfather II was completely movie-original, though.