Well, it never actually SAYS what happened to him. If I recall wasn't he starting to write a book or something about it? While living as a hermit in a cave?

In all honesty that was the part I *REALLY* hated about the book, above and beyond the rest of it. The ONLY enemy of the Corleones to EVER survive is this unknown Geraci guy, and why? Because Winegardner wanted his own character to look like a badass and not die like everyone else. Complete garbage if you ask me. That's like someone writing a Harry Potter novel, for example, and having an original character defeat all the bad guys in the end, instead of Harry. It's deliberately breaking a plot to make your own character look better.