Expanding on what I said previously last year, it wouldn't have been as bad if it had been done like the Star Wars "Tales From" novels that took minor/background characters and gave them life. As I recall the books were based on the novel, and not the movies, but he could have done it a lot better if the first book had expanded on characters from GFII (e.g. Pentangeli, the Rosatos) and the second characters from GFIII (e.g. Zaza and The Ant, Altobello) to fill in the gaps with their background. Instead he went off on entirely his own tangent, with new characters and ignored existing characters entirely and had characters act totally different to how they acted previously. Basically the books should have been the filler between each movie, and ended just before the movies started (so the first book would have ended with the Corleones preparing for Anthony's communion, which would segue into the beginning of GFII, and the second would have ended with Michael learning that he's receiving that award from the Church).

The books would have been more well received if it had ONLY focused on his own characters and then made some vague references to the Godfather (e.g. have one of his own characters talk about "Don Michael's operations in Nevada" or have a cameo appearance in a scene).