Here's where Winegardner's novels fit into the saga:

The Godfather (1945-1955)*
The Godfather Returns (1955-1962)**
The Godfather Part II (1958-1959)*
The Godfather's Revenge (1963-1964)***
The Godfather Part III (1979-1980)

* The original novel and second film also cover the early life of Don Vito (c. 1901-1939) Note: In Revenge Winegardner says 1910 rather than 1901.

** The Godfather Returns also covers the early life of Michael Corleone (c. 1920-1945)

*** The Godfather's Revenge also explores the early life of Tom Hagen (c. 1920-1922)