It can't be any worse than the Winegardner books, and I'm sure we'll all read it. But I think the description of the book may be incorrect because we already know how Vito got to be Don Vito.

My guess is that it's going to cover the "lost years" in between the war of the '30s (covered in the novel, where Sonny took control of a regime), and Connie's wedding in 1945.

It's roughly a ten or twelve year period, so there's plenty of time to cover in book form. If it's really based on an old Puzo idea, it should be worth reading on that basis alone.