It's not a bad book -- but it IS a pulp novel, Puzo would absolutely admit that. The fact that a decent, but by no means extraordinary book, would go on to be the basis for one of the greatest films of all time is such a rarity (Gone with the Wind is probably the only other similar situation and that novel was probably better critically received in its time than The Godfather was, although it has understandably fallen out of favor today), that it is notable.
Adaptations are tricky and although it is very common for a sublime piece of literature to become a mediocre film, and somewhat common for a fantastic book to become a fantastic film -- it is rare for a film and a screenplay that are inarguably among the greatest of all time to come from a book that would likely not be memorable without the film.
Everything about The Godfather should have been a failure. From the casting, to the production, to the monetary situation at the studio, to the lawsuits, to the source material. But it's a near-perfect film. And that sort of anomaly coming out of such chaos (again, Gone with the Wind is probably the closest similarity when it comes to English-language film) is almost as remarkable as the film itself.