Read it. You will learn things about the characters that you wouldn't know otherwise. Plus, there are lots of interesting parts in the book that are not included in the movie. Some are good, like the parts about the furnace repair scam with Sonny, or the handling of the Capone hit-men by Luca Brasi. Others are not so good, like the time spent on Dr. Segal and Lucy Mancini. There is another character, Nino, that is Johnny Fontaine's buddy that doesn't appear at all in the movie.

It is a great book.