If anyone has read Accelerando I think the Singularity culture described in that would make for an interesting xeno culture out in the Halo Stars. With bandwidth limiting their travel, it'd be easy to see why they wouldn't spread beyond their native sphere of influence. However, because they trade in sentient beings as currency they would present a suitable danger to the acolytes. If they could be bargained with, why what Inquisitor wouldn't sell their soul to have the computing power of an entire star?

Alexandro Jodorowsky's SF-series (drawn by Moebius, Zoran Janjetov, Carlos Ximenes et. al) especially The Incal series and The Metabarons have been a direct influence on Warhammer 40K. These series are classic and a must read.

I second the suggestion for Richard Morgan, especially the Takeshi Kovacs series (Altered Carbon, Broken Angels, Woken Furies). Takeshi Kovacs is an UN Envoy which is part investigator plenipotentiary, part ruthless supersoldier. Altered Carbon gives a good picture of the fear and grudging respect an Inquisitor-level investigator commands among the elite.