The Godfather is a collection of every great Mafia story packed into one. Or so I think. For instance, THe opening wedding is based off of Joe Clombo's wedding. Coincidentally Al Martino(the dude that played Johnny Fontane) sang at Joe Colombo's wedding. Johnny Fontane is supposed to be Frank Sinatra.
As far as theme goes, Mario Puzo said it was about a king and three sons. Michael was the one that wasn't going to get mixed up in the family afairs. Sonny was the one eager for the throne, and Fredo was the idiot son who just kind of existed. If you're looking for character changes like I have to do when I do a literary analysis, Michael was a patriot and a law abiding citizen. The turning point, the murder of Solozzo. Afterward he slowly gre colder, until he met Apollonia. After her death he grew relatively loveless and even colder. By the end of the book, he was all business and was no longer the innocent kid his father once wanted him to be.