This is the most psychological book of the series, and it's a good thing. You get to see what makes Fulgrim stoop so low, and become a slave to his base desires, bringing his whole Legion down with him. This book is also the first in the series in which we see one of the major battles of Horus Heresy, Massacre at Istvaan V. That battle alone perfectly shows the disgusting and hideous degeneration of the Emperor's Children Legion.
That scene where Marius Vairosean chops a Iron Hands space marine's head off, than proceeds to kiss his dead lips... That scene seriously creeped me out. Goddamn Slaneeshi perverts.