Yes, the novel ends where the first 'Godfather' ends. 'Pt.II' was the creation of Francis Coppola, using bits and pieces from the novel. All the Michael scenes in 'II' were written by Coppola and fleshed out by Puzo. Puzo never intended 'The Godfather' to be any kind of continuing saga, just a story he could cash in on. He was broke and his last novel, 'The Fortunate Pilgrim' was a small success. This book had a minor story about the mafia, a publisher mentioned to Puzo he should've added more about the mafia in that book. Puzo wrote a rough outline of another book, which became 'The Godfather', based on this publisher's suggestion.
'The Godfather, Pt.III' was purely a Coppola/Puzo invention for the movies. It had no relationship with the book at all.