Now you're kidding, right? A lot of the books focus on the Astartes. The vast majority of them focus on the Imperium, however, a few of them do not. I mean, we've been talking about the Eldar literature for a while now. I cannot recommend the Astartes books effectively as I'm not familiar with them and a lot of them are hit-or-miss. Nonetheless, I can help you with regards to Imperial Guard, Eldar and Inquisitorial works.