Mostly everything's been covered here already, but one major difference between the book and the movies which I would remark upon would be the existence of Vincent Mancini. In the book, Lucy Mancini's life after Sonny's demise is spelled out in great (I would say too much) detail, and it's terribly obvious that she wasn't carrying Sonny's seed when he kicked off. In the film trilogy, this obviously isn't the case. Presumably, in the universe of the films, she was never shipped off to Vegas to become a semi-big deal at the casino, but instead stayed in the NYC area and raised old Vincenzo on her own.