I don't know if this has already been discussed, and I may be wrong on a few things, but after reading Winegardner's books a few things didn't make sense to me.

1. In Part II, Francesca Corleone asks Michael for permission to get married, and its not to Billy Van Arsdale.

2. In Part III, at the party for Michael in the beginning, it doesn't appear that Johnny Fontane is married to Francesca (side note:Francesca should be over 40 years old at this point but is wearing the same dress as her twin sister)

3. I thought that at the beginning of Part I, the bosses of the Five Families were all whacked, yet in Revenge Cuneo and Stracci are still alive.

I'm probably wrong on some details here, but if I'm not, it doesn't seem as though Winegardner tried to stay true to the films.