Some are major events that it's shocking the show didn't cover (like Slade's family). Blood & his cult, Slade's family, the mask on the beach, things like that. The real thing to read it for is the explanations and stories behind a bunch of stuff the show never got into. It's better than a straight synopsis, but not by much. It reads very matter of fact with no flourish, and there's huge sections - the majority of the novel in fact - that just gives you characters discussing the events of the season.
So overall the quality of writing isn't very good. What I think happened was he essentially just told the actual writer the current canon answers to some lingering questions. Yeah one of the writers/story editors co-wrote it. I have read 'Vengeance' but not the Flash novel yet.
