This is the twelfth book in the Gaunt's Ghosts series. You can read my review of the rest of them here. It can be read as a stand-alone, unlike, say, Only In Death, but I don't recommend it as the series does have arcs. And this review may spoil the earlier works.
Takes place two years after Only In Death, so the emotional trauma they suffered in that book has at least settled down. (This is a less dark book than Only In Death. Then, that is not difficult.) Gaunt is, for instance, coping with his new eyes. On the other hand, they were taken off active duty and put into garrisons far behind the front lines, and the Ghosts are climbing the walls. They aren't suited for garrison duty. In fact, Rawne and a few other Ghosts -- including Daur -- get into serious trouble and end up behind bars.
Gaunt is summoned to the fortress they call Section. And learns that it is because they have a prisoner. A prisoner with very valuable information who refuses to talk to anyone but Gaunt. He proves to have known of him from his work on Gereon. The Inquisition Does Not Approve of the prisoner's even being in the hands of the Commissariat.
Meanwhile Chaos forces have inserted themselves into the planet, using plentiful witchcraft to hide and to hunt. They are after the prisoner, and their attack makes it into the fortress itself, but Gaunt is near the prisoner and gets away with him. And Wes Maggs, acting as his driver, actually gets them on the road. Rawne and his team escape in the confusion -- one of the Chaos forces came to shoot the prisoners -- and regroup. The Ghosts hear only fragments of the news, and ready themselves.
So, a mad, five-way chase to figure out what's going on and who can be trusted and what to do. In a snowstorm.
The parallels between this and Traitor General are -- interesting. On the other hand, the team sent after the prisoner is stronger, and the defenses against them weaker than in that book. I think it has something to do with which side is which.