Welcome, Qckrun! If you haven't seen any of the films yet, are you ever in for a treat!
"The Godfather" film follows the novel from 1945 through 1955. "Godfather II" has flashbacks to 1901, when the child Vito's family was murdered and he fled to America, and his years as a young man, through his killing of Fanucci. All of that was covered in the novel. GFII does not cover his rise to power, and Sonny's rise, which were in the novel. However, the film does provide a great sequence when Vito returns to Sicily with his family for a visit (no spoilers!). Most of GFII is concerned with Michael Corleone and events om his lifefrom 1958-60--years that were not in the novel. GFIII picks up Michael and his family in 1979--none of the stuff in GFIII was in the novel.