Just idly wondering here: I wonder if Winegarden was under some kind of contractual restriction from Paramount, or FFC, or the Puzo estate, or whomever actually controls copyright on the character, as to how much he could involve Michael Corleone in the plotline.

Both books ("Returns" and "Revenge") dwell conspicuously on subplots involving secondary characters (Fredo in "Returns," Tom Hagen in "Revenge,") and BOTH deal quite extensively with the new character, Nick Gerace, created by Winegarten strictly for the books -- while leaving Michael Corleone almost a peripheral character in the storyline in both books.