But.. the novel explains that timeframe. the third section of the novel ends with something about everything being peaceful until the attempted hit on Vito (i.e. after GFI begins).

I heard that this novel will be Godfather Part II, but Part II following the book (i.e. Michael has two sons not a son and a daughter, Connie remarries a good Italian guy, the only dons killed were Barzini and Tattaglia and not all the family heads, etc. etc.) instead of the way Part II unfolds in the movies.