Everything is subjective in these cases. I enjoyed the novel because of its' ability to expound further on the characters and situations. It is to be used as a companion to the film. Puzo himself said in the Godfather Papers that he did not want to write this book, but he was broke and the publishing company that was advancing him substantial amounts of cash wanted him to write a 'breakout' novel. The story is not truly fiction, it is based on the families and events of that era, and yes that does include Frank Sinatra.