A sign of a good story is when the author gets you to identify and symapthise with the characters. Considering this tale is about a legion busily sliding into heresy by pursuing dark knowledge and defying The Emperor's commands, Graham McNeil makes it easy to believe how one small step leads to another, and a whole legion strays from the path of right while utterly convinced of their own loyalty. It's the Space Wolves, sent to Prospero to enforce The Emperor's punishment, that appear as the forces of anarchy and barbarism that the Sons are so determined not to become.
Despite dealing with the subjects of sorcery and the Warp the book is still firmly rooted in the philosophy of the 31st millenium. The Sons view their powers as an extension of science, and the way they term their bound warp-spirits as 'tutelaries' neatly shows how these are sorcerors who don't believe in superstition or magic. Other Heresy books have sounded rather too '40K' in the way characters go on about witchcraft and demons. Like Dan Abnett's Legion (The Horus Heresy)
, it also becomes apparent that there are reasons other than blind loyalty to Horus why a whole legion might turn its face from the Emperor's light. Terra's history before the Great Crusade gets some coverage, enough for me to think that a Unification Wars series would be a viable prospect once this one has run its course.
An excellent book, the fact that Dan Abnett has written its partner from the other viewpoint gives me something clear to look forward to in the series.