I'm a pretty avid reader of Black Library fiction, being a fan of the 40K universe in addition to the game. I get a lot of my Black Library books from paperbackswap.com. That's where I got this copy of Dan Abnett's 'Xenos'.
The series was highly recommended by gamers at the FLGS, and I was running low on my backstock of novels. I decided to start reading the series, despite my love-hate feelings toward Inquisitors in general.
The story starts out in a gumshoe detective fashion, which I found to be quite good. I'd not really thought of Inquisitors as detectives, but more as warrior-psykers or covert spies. As the book goes on, you learn that the detetive vibe is because of Eisenhorn's personal style. There are several Inquisitor characters, each with their own methods and attitudes. You get to see some interesting play between the Inquisitors of the different schools, from extreme radical to fanatical puritan.
The book's pace is good, though there are a few points where it slows down for exposition and introspection. Thankfully, the pace stays brisk overall, whether there is an action scene or not.
The large majority of events in the book are believable within the fram of the 40K universe. Eisenhorn isn't a one-man army. It's important to note that the background details are of the older GW canon. Example: Deathwatch Marines are referred to as a Chapter, not an organization. There's no reference to the Marines being from individual Chapters at all, which is fine. It may even be possible that Abnett glossed over those details, or misrepresented them, because Eisenhorn simply doesn't know them. The book is written in first-person perspective, after all.
There is also a believable passage of time, versus character development. For example, the characters make a 30-week journey through the Warp on a ship. They spend their time constructively, and layman/amateur characters manage to evolve into capable skill users. Those are small details that make the story work.
My only gripe abut the book is the ending. The final climax is ended by a bit of a deus ex machina. I'm aware the book is the first part of a trilogy, but the heroes get off way too easy, with the Big Bad simply running off at the end though no actions of the main character. It's my one pet peeve in fiction writing. Solve your characters' problems through their own actions, authors! You don't end a story with a minor character saving the hero with a small act.
All in all, I'd give the book a solid A grade. The only thing keeping it from an A+ is the ending, and the fact that said ending is a lead-in to the sequel.
The entire trilogy is available from the Black Library as an omnibus now, as opposed to the three book format in which I acquired it.