Welcome to the boards. The only parts of the Trilogy that are also in the book are all of The Godfather and the flashback scenes in Godfather 2.

The book gives you more insight into characters such as Luca Brasi and Mama Corleone. There are parts of the bood that were (mercifully) left out of the movie, such as what happens to Lucy Mancini after Sonny's death and lots more about Johnny Fontaine.

However, Puzo is an excellent storyteller and I do recommend reading it. It will make the films more complete.