I saw the film first and read the novel several years later. In other threads, it was discussed how Puzo liked to throw in stuff to demonstrate how smart he perceived himself to be. I would agree.

Johnny Fontaine & Michael Corleone were essentially co-lead characters in the novel. The Fontaine plotline was wisely left out of the film version by Coppola.

Like most, I typically favor the book over the film. But in this case, I much preferred the movie over the novel. That's not to say it was a bad book, but there definitely lulls in the novel.

To add to what its da jackeeett said, I thought the background about Luca Brasi helped flesh out why he was so feared. Puzo made a good parallel between Brasi and his heir apparent, Al Neri.