The third and last novel in the Ciaphas Cain: Hero of the Imperium omnibus. Which also contains the short story you should probably read first.
Cain is still with the 597th Valhallan, and this time, they are going up against the forces of Chaos. In fact, they are dragged off their latest assignment before the mop-up work is done, which tells them that it's crucial. Much to Cain's displeasure, their travel ship also carries a regiment whose commissar is an annoyingly by-the-book and pious classmate of his, who's managed to be shot because he's found a regiment as obsessed with the emperor as he was.
They arrive in the midst of the fight: Chaos cultists attacking a disembarking regiment. Cain has his shuttle land in a vegetable garden and go to the attack. Which leads to a frantic attempt to root out the problem, punctuated by occasional attacks on them and their raids on discoveries, while they wait for the invasion.
It involves infighting, a discussion of promotions, filtering information about explosions out of data, a genuine Space Marine (in fact, several), some ghastly cult rites, and Cain's being arrested in the middle of battle. Great fun