The Horus Heresy Warhammer 40,000 novels have always been about expanding some basic lore concepts known since long: The Emperor created 20 space marine primarchs, the forces of Chaos spread them through the galaxy, The Emperor reunited them, at a certain point some, led by the favourite and warmaster primarch Horus Lupercal rebel, and although in the end are defeated, slain some loyal primarchs and leave the emperor heavily wounded and destined to sit forever in the golden throne.
After more than 50 books on the general story arc, Games Workshop/Black Library (their editorial wing) decided to elongate the money making machine a bit more, and divide the final confrontation (the Siege of Terra) in apparently another further 9 titles. Don't get me wrong, lore is the single best thing about the Warhammer 40k universe, but at times is clear the lore expansion is secondary to adding more volumes.
Although this novel is the 8th title on the Siege of Terra series, I wanted to begin with it because it's the most critical point: Loyalists are losing the war, the enemy is almost at the palace gates, reinforcements are not going to arrive in time, and the Emperor must do a final gambit to turn the tides: Go out and fight Horus.
So I'm probably missing a lot of context on some of the characters, and I indeed felt some of the side-stories felt a bit lacking information, so that's on me. But being a long book (more than 650 pages according to my eReader), and considering there will be a second volume (I sincerely hope they don't go too far and make it 3 parts), while reading I got the impression that there was a significant amount of fill content. The story advances, but at slow and very deliberate steps, also leaving you with a multi-faceted cliffhanger everywhere.
The writing is excellent, and having a lot of small chapters, each focusing on one of the characters or "groups" helps to keep clarity, but my impression was that it is intertwining, twisting together both three really important story lines (the Emperor, Malcador, and Horus) with lots of, at least for now, weird secondary quests and adventures, some of them a bit odd and even a tad silly.
Still, I think it's a good read, and combined with the remaining volume will become the new expanded lore about that truly important moment in the WH 40k universe. It could simply have contained the whole story in a single title.