I have actually just completed the aforementioned novel, which is from the point of view of one of the Grey Knights, who are, apparently WAY different from the other Astartes. One of the things that made me think they were a specialization within each chapter was the fact that the game refered to them as psychics and I didn't think there'd be a whole chapter of psychics, but apparently they are.

Hyperion, the PoV character, actually has a connection to Ravenor and the events of that trilogy, which was surprising and gratifying to fans. He fights with demons on the world of Armageddon (which threw me for a minute as a I had already read a book by the same author about the Black Templars fighting Orks on Armageddon, but it seems to have been two different wars, with the characters referencing a previous Ork war there as well, and the Space Wolves & Grey Knights fought the demons before any of that), and the clean-up work after the war causes a conflict between the Space Wolves and the Inquisition, with the Grey Knights sympathyzing with the Wolves, but forced by their own circumstances to go along with the Inquisition. Far from my initial assumptions about the role of the Space Wolves, I came out of the book admiring them far more than the Inquisition, who are the antagonists and prove themselves capable of things even more horrific than any aspect of Eisenhorn's fall from grace.

When the Space Wolves request the aid of the Grey Knights in fighting the latters' specialty foe, they took a lot of precautions in asserting their control over the theater (a trait alluded to in Dan Abnett's HH novel, "Prospero Burns" ), and limiting the actions of the Grey Knights and their exposure to the war in general. At first I thought this bespoke tendencies and tactics that in an Inquisitor who be labeled radical or heretical, but it turns out that the Space Wolves were accurrately anticipating the extreme and drastic measures the Inquisition and the Grey Knights take to quarantine demonic exposure. By the end of the book, I was wishing the Space Wolves had existed around the time of WW2 to put Churchill, Marshall and all the other executors of Operation Keelhaul in their places (those places preferably being "splattered around a room as a result of taking a bolter round to the face" ).

It wasn't quite a case of Space Wolves vs Grey Knights, as the Grey Knights torn between what some perceive to be the letter of their obligations and others to be the spirit of their duties to the Imperium. The issue at stake is arguably valid on both sides of the equation as well. Where moral issues in W40K seem to mostly consist in wallowing in the fact that this incredibly tyrannical and oppressive government is all that stands between the human race and anihilation, in this book, the author finds a legitimate case of an ethical gray area, and two conflicting sides that both have a good claim to be in the right. The Grey Knights, rather than one of the sides in the conflict, are closer to being the battleground on which it is fought, even if the Grey Knight PoV protagonist largely lacks agency in the conflict itself.