Note: I misunderstood this was a full fourth book, but it is instead a short story, part of the Victories of the Space Marines novel.
This short tale on the surface appears to be telling about Alaric's squad of Grey Knights' assault of an imperial ship captured by a chaos fiend, but really what it talks about is how the process of becoming a grey knight happens. From how the storm bolter bullets become sacred, to how their armour gets the magic wards, or how a simple human becomes an inquisitorial interrogator.
All of them share one common denominator: huge sacrifices.
Recommended read, I enjoyed it a lot.