A game guide to include: a detailed walk through for the single player campaign including locations for all relics, a history of the Genestealers and the Deathwatch Chapter along with concept artwork. An example would be the WARHAMMER 40K Prima Official Game Guide by Raphael van Lierop, the Game Director.
Unfortunately, I don't think we have the resources or a sufficient potential customer base for a thick, paper guide.
If you're looking for tips and guidance for the game, the community has crafted some pretty solid guides on Steam. I'd recommend checking them out.
The wonderful people at Focus have also posted here a bit of fluff on the Angels of Vengeance and the Guardians of the Covenant.
Lastly, the Deathwing website has a "Media" section where you can find HD visuals and wallpapers.
It's not a paper guide, but I hope this makes up for it for now
If you're curious about Deathwing Lore, there is a paperback book that'd be up your alley - https://www.amazon.com/Deathwing-Warhammer-000-David-Pringle/dp/0743411714
There are also quite a bit of lore videos about 40k in general on youtube