Spoilers ahead. Things I found of note in the book:
P.108. The Death Guard ritual of drinking poison with Mortarion. They refuse to call it a ritual, but in a similar way to the Mournival and their water/moon ritual it seems that the Emperor’s attempts to stamp out ritual and religion have been insufficient, even among the Astartes.
P.169 – Garro describes the wargear of the Dusk Raiders as the dull white of old marble, but with the right arm and shoulders coloured in a deep, glistening crimson to represent that they were the Emperor’s red right hand, the “relentless and unstoppable”.
P.226 – the Choral City defenders wore camouflage, but it was not effective due to the image intensification lenses and infra-red prey sight function of the Astartes helms.
P.373 – a comment on the Warp, and how Navigators and astropaths experience it differently – as math, or sound.
Temeter and Huron-Fal’s last stand is awfully poetic.