The book ends shortly after Michael arranges the murder of the Five Families bosses.

The "prequel" track (Young Vito) of GF II is based in large part on chapters of the GF novel which weren't inlcluded in GF I. The "present-day" track (1958-1959, Hyman Roth, Cuba) was written from scratch.

GF III was pretty much written from scratch by FFC and Puzo.

There are still a few sections of the novel, mainly dealing with the Corleones' rise in the underworld hierarchy from about 1928-1940 and Sonny's early days in the operation, that are generally assumed would be included as part of the plot line of a possible GF IV.