As has been my habit with these, after finishing Book 3, I quickly moved on to Book 4. While Galaxy in Flames tied up the Loken trilogy, it also hinted at the importance of the divergent story arc surrounding Nathaniel Garro. Just in case you want to read back, previous reviews for Book 1 and Book 2 are linked. As before, I will try to largely avoid spoilers, but we all know generally how things will go.
I hadn't been all that interested in the Death Guard Legion going in, but I was quickly absorbed into the story. Garro is a very similar character to Loken, though maybe with some more allies than Loken seemed to have. The strength of this book lies in the development of the secondary characters. Where in the opening trilogy, there was the main cast and all their minions were more or less faceless, here, Garro has trusted soldiers, lieutenants, a servant, and the strongest non-military characters in the form of Sinderman and Keeler supporting this story.
While the Flight itself doesn't take place until well into the novel, it does provide an interesting alternate view of the Istvaan III conflict.
Another strength of the novel was that it repeatedly had instances that show just how fine the line the characters are walking between Heresy and Loyalty. The most glaring of which is the encounter with Rogal Dorn, which shows an actual balancing act on the verge of fall before Dorn's recovery in the face of reason over emotion. These scenes deliver a much better level of believability to the turn of the Primarchs, even if Mortarion's fall isn't explored in the least - he's assumed to have already been turned and acting with subterfuge.
Skipping much of the action, the novel concludes with tantalizing hints at what lies ahead for Keeler and Garro. In particular, there is the implication that he will form some sort of fighting force to oppose Chaos. Theorize where you will, it's a nice bit of hope and interest in a continued story arc (that I believe has been largely left shrouded in mystery rather than explored) that fits within but steps beyond the inevitability of failure that was prevalent in the Loken trilogy. This note of hope is particularly refreshing after that tragedy, and had me ready to move on to the next part, even if I would have preferred to continue following Garro.