Spehss Mahreens being political? And not just in the "Blow up the planetary capital and claim the world for the Emperor!" way?
The fun thing about Space Marines is that it's easy to turn them into Spehss Mahreens - big dumb armour-clad head-chopping grunts who happen to have some really cool weapons. But at the same time, they can be turned into forward-thinking political battering rams who also happen to be good at chopping heads.
In a long series like this, no one author does all the work; they build on what previous authors have written in the preceding books. David Guymer has done this exceptionally well, however. Descriptive without being florid, political without assumptions towards sci-fi Machiavelli, and even-handed in his treatment of humans in combat compared to the Space Marines. He's written probably the best ship-to-ship combat scene in any of the Black Library books I've read, keeping things confusing enough to remove the "omniscience of the Emperor" so many naval battles use, but at the same time keeping the action flowing and tossing in some very interesting tidbits on the origins of the Beast. The inclusion of an elite human combat group that can (at a distance, at least) hold their own against the orks and impress even the unimaginative and stolid Fists Exemplar is a nice shift from the general incompetence of humans to this point in the Great Ork War. And the Iron Warriors keep their pragmatic, evil/neutral mien, being connected to Chaos because of that pragmatism rather than any Word Bearer-style idolizing, which is a marked contrast to how so many of the Chaos Legions are portrayed.
All of this is done while keeping the impetus of the Last Wall on Terra rolling, although with some unexpected actions happening. The Inquisitorial power struggle and council dynamics take a back seat (while staying relevant) to internal moral dilemmas of the Space Marines. There's much more focus on the Fists Exemplar than any of the other chapters in this book, although the Black Templars do get some page time (and set up some interesting contradictions in the Black Templars of M31 and the Black Templars of M41).
Very well done, and my favorite book in the series to date.