Book: The Godfather, Mario Puzo, 1969
Movies: The Godfather, 1972, The Godfather Part II, 1974
Seldom have the cinematic versions of an iconic book exceeded and enhanced the printed work. That may very well be the case for The Godfather.
The source material is fantastic. Mario Puzo wrote a work of fiction based on events and situations he might have heard about growing up in Hell's Kitchen. There are certainly plenty of parallels between scenes in the book and real life. The life of the fictional Johnny Fontaine character shares many similarities with Frank Sinatra.
What was so great about the movies was that Puzo and Francis Ford Coppola did such a great job of culling and rearranging the parts of the book to make two incredibly cohesive movies.
The key to any great adaptation is the ability to leave in all of the important things while eliminating that which can easily be done without. That is certainly the case here. Whole sections of the book, while being important to the story on the page, were condensed and streamlined on film.
For instance, there is a lot more information and history about Fontaine in the book than what we see on screen, but the movies are done in such a way so the audience doesn't miss it because the pertinent parts are still portrayed.
Sometimes, the greatness of the Puzo's written work is lost because of the fame and popularity of the movies. If you ever have the chance to read the book, you will be amazed at how closely the movies follow along the book in all the right places.