I've recently finished the latest Horus Heresy novel by James Swallow (writer of Flight of the Eisenstein, another fantastic part of the series), Nemesis. It covers a plan by the Imperial assassin cults to send a team of assassins, one from each temple, to assassinate Horus. Of course, a plan is simultaneously put into motion by the traitors to send a vicious assassin to kill the Emperor.
I'll start out by saying that I really liked this book; it's a great addition to a great series. Sure, you know from the beginning that neither of the assassination plots succeed, but that's not really the point of the series. The main interest is seeing HOW it unfolds.
The assassins are covered very well, firstly because you get to see them in action. As the execution force is assembled, the group goes from location to location picking up assassins immediately after their previous missions have completed. Doing so, the reader gets to read about the missions while they are happening, to some effect. Sure, reading about a Vindicare sniping people isn't very interesting (and it's a little corny in the book), but reading about a Culexus (the anti-psyker) assassin in action from the Culexus' point of view was very interesting.
Also, you get to feel that the assassins are people, with real personalities (somewhat) and feelings and such, and you get to see how they interact with one another and how they view one another. They aren't portrayed as single-minded killers. It's especially interesting to read how the Eversor thinks when the focus is on him... ok, maybe one of them is a single-minded killer...
Finally, the book includes an operative from the Venenum and Vanus temples, who aren't really mentioned often in the 40K background otherwise; I think only in the Assassins codex from 3rd edition. They seem to be in there as plot devices, though, as opposed to being there for the necessity of their skills (although the Vanus obviously contributes quite a bit to the mission).
I don't have many complaints about the book, but there are two issues I have. First, I think the book jumps around too much, and not necessarily to the story's benefit. Generally, moving from one character's viewpoint to another can really help drive the story, but only if it's done for that goal. I don't feel that Nemesis is really written to fit with such a goal. There are two separate stories that are going on inside it, and although they are related, the jumping between the two probably should be done a little less frequently. Of course, that's my opinion, and I'm not a professional author.
The second one is very minor, and is more of a confusion than a complaint. There are two parts earlier in the book (one at the beginning, and one in the middle), where Horus could be involved, yet it seems like he's purposely kept apart from the action... then some of the characters directly interact with him at the end, and it really doesn't seem like there was a reason for it. It's probably solely because he wasn't really relevant to the story at that point, but it stuck out at me.
So there are a few things I want to also talk about, but of course, if you haven't read the book, you shouldn't read them. So stop reading if you only wanted a review!
**** SPOILER ALERT ****
The book gives a very clear hint that someone warned Horus about the assassination attempt, and even the First Chaplain is surprised that Horus was aware. There's some suggestion that it might be a spy in the Imperial Palace... but I think the implication is that the Chaos Gods are whispering directly to Horus. We know that when he assaults Terra, he's going to be significantly 'enhanced' by the Chaos Gods' gifts, so I suspect this is the start of them showing their favor to him. It certainly will make future dialogue/interactions with Horus more interesting.
The response from both the Emperor and Horus regarding assassins is one of the final parts of the book, but is very interesting. The Emperor declares a change from the Assassination clades to a more governed form (presumably the Officio Assassinorum), but defends the need to keep such a group because 'in the war to come, every weapon in the arsenal of the Imperium will be called to bear'. Horus, however, describes assassins as 'a tool of the weak'. I just found that rather fascinating- the noble Emperor using a backhanded means to try to end the war, with the traitorous Horus decrying a similar attempt by one of his allies.
I'm very excited to read the next books in the series- I can't wait!