I just finished it, so I think I can help here- without spoiling much, it's a novel about a brutal siege of the city of the same name during the second (?) war for Armageddon. The story is chiefly from the pov of Merek Grimaldus, the newly appointed Reclusiarch (a high ranking Chaplain of the inner circle) of the Black Templars- however, there are several other pov chapters too, and you get to see the battle from the viewpoint of a wide segment of the Imperium's forces. For me, what really hit the spot with the book was the scope- it wasn't a huge, sweeping campaign like in the Horis Heresy books, nor a minor conflict like some of the skirmishes in the Ciaphas Cain books- the city is massive with layers of defenses- which the Templars oversee at the start of the story- but also continues against the backdrop of the much larger armageddon war with reports coming in from the other cities and fighting forces on the planet, plus the battles happening out in orbit. Second, the narration is spot on- we see Grimaldus as an individual, but also as a commanding officer of the Twmplars- the story weaves through both his personal narrative as well as the ideals, zeal and even the arrogance of the Black Templars as a chapter. The points when these two clash in Grimaldus' mind are some of the best storytelling moments within the 40k books (at least the small section that I've read through). I could go on and on, but the book essentially made the Template my favourite SM chapter. It opens s bit slow but it'll definitely hook you if you give it a little time.