It is one of the most enjoyable books I have ever read (not sure if this is because I like the film so much) but is by no means most well-written book. I believe The Godfather could have been a lot better had Puzo put more effort into the actual writing it. I do realise though that he said many times that he was broke for money, and the sole purpose of writing it was to make a bit cash. He could never have foreseen how popular it would become.
Mick