Halo: The Fall of Reach is one of the most praised game novels, if not just in the Halo universe, but ever. It's one of the first things I hear recommended when I see people asking for suggestions.
I suppose that is why it's not surprising that the book is seeing an animation made for it. We finally have been given a trailer for how that will look too. Halo: The Fall of Reach will be a three part series that will launch alongside the game. It will come with the Limited Edition version of the game and also on the Halo Channel to buy separately.
It covers the story of Master Chief where he came from, as well as getting into into who Blue Team are, a huge part of Halo 5: Guardians.
Take a look at the trailer here.