Ciaphas Cain book 10. No major spoilers for the earlier books, I think.
Our favorite commissar is back with his usual ambiguity as to exactly how much he is underselling himself. . . .
It's an adventure with the 597th, and they are sent off to an ice-world to deal with eldar pirates. A world with a lot of tunnels, which is actually what Cain likes best, but which does result in his being sent off to chase down unmapped ones. Where they find a cult summoning a daemon. . .
It involves gems that the eldar prize, a maze, tunnels on another world, the appearance not only of Amberley but also of another inquisitor, how to trace smuggling between worlds, a problem Cain's faced twice before, a lot of fighting, and more