While I have not read all that many of the books in the enormous Warhammer universe, I am convinced Dan Abnett comprises their best author. Eisenhorn works as an Imperial Inquisitor, travelling around the worlds of humanity and stamping out heresy. It seems much of the heresy comes from the Chaos associated with the Warp. I read the Ravenor omnibus a bit ago, which basically follows the same path as Eisenhorn a few decades later; indeed, Ravenor himself studied under Eisenhorn.
Xenos starts with Eisenhorn and his team landing on Hubris, an icy planet currently in its deep freeze zone (Hubris has a highly elliptical orbit). Eisenhorn and company are hot on the heels of a 'facilitator' who works for heretics that they have been tracking for several years now. Well, after some major excitement, they find and kill him, but obviously, he was working for someone else. The rest of the book we follow Eisenhorn backtracking the heretic to find out his employers to deal with them.
Abnett writes well and Xenos reads like a mystery thriller set in the 40K universe. Abnett takes us on a tour of the seedy underbelly of the Empire here, starting with Hubris but Eisenhorn's path takes him to several other planets. The more Eisenhorn and team dig, the bigger the conspiracy seems to be, with many highly placed members of society and even the Imperial Fleet. Eisenhorn's team consists of his pilot/tough guy Midas, Aemos, an old, learned scholar, and two new additions-- Bequin, a 'null' he picked up on Hubris and another enforcer also from that planet, Fischig. A 'null' exerts a psychic shield of sorts, disrupting other's psychic powers (Eisenhorn has some psychic powers of his own). It was fun to read about the team gelling and fitting into their various roles.
I loved the atmosphere here, seeing how the 'real people' live and work. Abnett also has a gift with pacing, keeping the story moving right along. Finally, the aliens he encounters were really spooky, especially as they seemed to exist in 4 dimensions. Whew. 4.5 Inquisitor stars, rounding up!!