A righteous case of retribution, personified by an appropriate agent. In this case manifesting itself, as an orrible **** - ME!
Snatch quote aside, the book is a really good read, and another fantastic addition to the Horus Heresy series of novels. Like a couple of others (Mechanicum springs to mind) this book focuses less on the legions, and far more on the humans of the Imperium. Admittedly, assassins are quite extraordinary humans, but they are human nonetheless. This book gives the assassins a human side I'd never before considered - though this of course is in varying degrees - the Culexus doesn't come across as all that human, and has an autistic quality to their character. But given the background, this does make sense.
Obviously this book has the major drawback that it shares with the whole heresy series - we know who the bad guys are. We know which legions fall, we know who wins. We also know that, whatever happens, the assassins will fail to complete their mission of killing Horus. But like most Heresy novels it brushes this aside. We know the Alpha Legion fall - Legion told us how and was a fascinating book. In this, we know they do not suceed - but it is still an excellent story that had me gripped to the end.
Speaking of the end, and please do not read any further if you have not yet read the book... or if you do so then don't blame me, I've given plenty warning! :oP
Anyway, at the end it was implied that Horus had been tipped off that the team was out to get him, and thus sent someone else to take his place. But who tipped him off? Not many people knew about the team. Is there a traitor among the leaders of the assassins? Personally I think not. My own opinion (and if you're going to disagree in the comments, please give the same spoiler warnings I have given here so people who skip this paragraph don't accidentally get the story from your riposte) is that the "traitor" if you will is none other than Rogal Dorn. He seemed so appalled by the idea of sneaky war, that while I do not think he would ever betray the Emperor or the Imperium, he may have tipped his brother off about the assassination attempt. Why, you ask? Partly because he thinks assassination is a dishonest way of making war. Mainly I think because he thinks Horus predictable and a preferable adversary to whoever would take the reigns of the crusade following his death. And possibly, despite it all, he wouldn't want to see one of his brothers meet such an ignoble end.
Anyway, those are my thoughts. All in all, an excellent book. I am eagerly awaiting the next installment of the Horus Heresy series.