The book covers most of the first film and has scenes from the life of young Vito in the second film. There is no part of the novel that goes into Part III, so don't bother seeing that.. it won't really enrich the book.

There is no particular order (like first and second halves of the book). Mario Puzo used his mastered ability to weld the different passages into just the right places to tell the story perfectly with flashbacks. There are also new parts of the story, with more emphasis on things like Johnny Fontane, Lucy Mancini, and middle-aged Vito's power struggles.