This is another in Sandy Mitchell's growing line of Caiphas Cain (HERO OF THE IMPERIUM) books. The basic idea is simple. Warhammer 40000 is not a nice place. In fact it is so over the top with grimdark hopelessness that it can often be viewed as affectionate parody, take that, and whatever else the writer wants to throw into the blacker than black setting.
This latest installment takes place between his adventures in Prosparity Wells and the planet wide Ork invasion he helped put a stop to, and his finding out a frozen world housing a city sized promithium processing plant under attack by Orks was also a Necron tomb world. Here he has just recovered from a misadventure ending with him as the sole survivor and is now a liason officer between the Imperial Guard and the Genetically modified Space Marines as they head off to smash a planetary insurrection.
This is complicated when Cain hits planetside, discovers the situation is far worse than it appeared on the way in and is put in charge of babysitting the planetary governor's daughter who fancies herself capable of leading men into battle (and has an honorary commission to make it legal.) Oh it doesn't end there, and it gets to the point where this initial setup is made to seem quite pleasant given the state things are at near the tail end, but it's all very believable in contest.
It's a fast paced book that I recommend even to people who think that the 40k setting is too depressing to enjoy. As with every other Caiphas Cain book this has more a casual adventuring feel than 'everything is on the line and there is no winning solution' tone that I have noted from other Black Library series.
Since this is not the first volume in the series one might wonder why I would recommend it. First off the series doesn't take place in chronological order. Secondly we are given generous footnotes throughout by one Amberley Vail, Ordos Xenos Inquisitor of the Imperium and heavily hinted to be Cain's occasional lover as well as collector of his personal notes in a 'not for the general public' collection of volumes detailing Cain's career as a Commisar of the Imperium of Man.