Well you see, Eric Nylund and bungie wrote "The Fall of Reach" and it was made canon by microsoft since at the time, they didn't have any plans for Reach. But when they wanted to go back and make a game, they ignored the book and redid it all, smashing the canon. Also, i don't recommend the Flood, it is terribly written and is essentially Halo CE but through a book's point of view.