You should read Harlan Lebo's definitive "The Godfather Legacy," which has priceless material about the making of the Trilogy. Most of us here regard it as the bible.
The Godfather Legacy focuses on Part I, en devotes 1/3 ot its pages to Part II and Part III. The Godfather Book by Peter Cowie, which for me is the bible, focuses on all three films.
"It was between the brothers Kay -- I had nothing to do with it."
