Can't answer that in a note, but the main differences lie in the depth of the story and the little details. While the storylines are "similar", in the book it's as if you're going into their lives and seeing everything firsthand, whereas in the movie you're seeing a finished product that a director and an editor created for you to see the overall picture. Like, in Godfather Part II, you see the beginning of the film in 1958 aswell as in the 1920s with Vito when he came back from Italy. In the book, it describes what he talked about at dinner (Like when he says he doesn't know who killed Fanucci when his friends ask him), or is talks about how Santino threatened him by saying he knew that he'd killed the old Don back in the early days. Then there's simply more meat it in, when it comes to dialogue. You see Michael say things he didnt in the film and whatnot. I highly recommend the book, it gives you a better idea of what the film is all about