I acquired a preview copy and started it the other day. So far so good!

To clarify, there is a timeline in the beginning of the book that shows where this book takes place:

The Godfather (1945-1954)
The Godfather Returns (1955-1958)
The Godfather Part II (1958-1959)*
The Godfather Returns (1959-1962)**
The Godfather Part III (1979-1980)

*GF2 also covers early Vito (1910-1939)
**The second half of The Godfather Returns also covers the early life of Michael (1920-1945) in flashback scenes

Hope this helps