This book was a fantastic read.
it is a breath of fresh air from the usual cardboard cut-out bag guys in all 40K books. Oh, you're a World Eater? Well, you must be a psychotic worshipper of Khorne, then. Not so in this book, some of them are still trying to philosophize their lot in the greater scheme of things rather than just screaming about skulls n stuff.
The main character, Iskandar Khayon (a strange last name, considering that is the name of a training discipline for the T'au), is damn near sympathetic, and merciful and generous to his crew. He is a fantastic balance of not mistaking a certain level of compassion for weakness. If anything, he is the scariest character in the book, when we learn what he can bring to the table in a fight.
I truly like that the author did not use GW brand names for the species in his book as blatant free propaganda and advertising: Khayon's ship is obviously crewed by T'Zangors, but not once is that name mentioned. He has what amounts to a Dark Eldar Wych as a psychic lover, but she is never called by her faction title.

I would highly recommend reading this book, if anything just to get a perspective from the other side that avoids the usual tropes, and this story is definitely imaginative in revealing that perspective.

*******SPOILERS SPOILERS SPOILERS***********************
The whole plot revolves around finding out that the Vengeful Spirit is still out there, hidden by Abaddon. Because the 'Sons of Horus' lost the siege of Terra (at least, this is what the other nine traitor legions think), the Sons are being hunted down and exterminated by the other traitor legions. Khayon is actually friends with a Sons Mournival legionary, and agrees to help him find the Vengeful in order to turn the tide of the legion War against the Sons. The only mainstream character from the 30K/40K 'verse we meet is Abaddon, everyone is a fresh new character (with a cameo from Fabius Bile, and a very short-lived clone of Horus), all of which ADB did a great job on fleshing out with their own unique personalities. At the same time, they come across as tragic personalities, too, to the point where I actually felt bad for some of them, and for the fate of the Rubricae Marines. (And, according to the chaos gods, Horus was expected to lose: the keep referring to him 'The Sacrificed King'.)
Basically, the book is about the formation of the Black Legion, their creed, and that they are not just fighting to survive, but to have a resurgence of what being in a legion meant, instead breaking down in to petty squabbling warbands. The Legion is not restricted to just outcast Sons of Horus, but any who swear to follow the creed are welcome.
There are two craftily veiled references to much older books, as well: the distillation of Psykers into a rare liquor, as originally written in 'The Eye of Terror', by Barrington J. Bailey, and a nod to the splintered mind of the Emperor, as originally posed by Ian Watson in the Inquisitor Draco trilogy.