The Emperor's Finest (Ciaphas Cain Book 7)

Commissar Cain is called to duty once more, saving a governor’s daughter from a planet over-run by rebels. The uprising hides something far more sinister however... The search for the source of the threat leads Cain to a drifting space hulk – a far safer place than beside the obsessed governor’s daughter. But when the Reclaimers Space Marines suffer devastating losses at the hands of the Great Devourer, Cain and his trusty aide Jurgen must go it alone. With the aliens waking and a group of stowaway orks on the loose, there are no safe places to run or hide, and Cain must use all his ingenuity and cunning to escape the space hulk alive.

Read it because
It's a typically sideways look at the mighty Adeptus Astartes (albeit a pretty unusual Chapter), viewing them through the eyes of the wry and dry Commissar Cain.