'The Godfather Papers' is not entirely about 'The Godfather', it does have a chapter or two about Puzo's memories of writing the book and getting involved in the screenplay for the first 'Godfather'; casting, meeting Coppola, the incident with Sinatra, etc.
The book also includes a lot of short stories Puzo wrote over the years. Basically, the book was a way for Puzo to make more money off the 'Godfather' phenomenon at the time. Nothing wrong with that.