Hello!
Welcome to yet another JBP review :-)
The Outcast Dead - by Graham McNeill - is one of those books that will leave most either hating it or loving it. I am more on the "loving it" band wagon. If you expect great epic battles, you will be disappointed. If you expect the Horus Heresy timeline to be moved forward, you will be disappointed. If you expect a read that will flesh out the workings of the Imperium, and get a look behind the curtain, then, well, you will not be disappointed.
When the "shell shocked" astropath Kai Zulane is returned to Terra after a horrible accident in order for him to get a 40k version of R&R, events happening around force a "terrible" secret into his unwilling head, and he finds himself hunted by the Imperium he is meant to serve and herded by an unlikely band of Space Marines, who's legions have turned traitor whilst they were serving as honour guards of sorts on Terra.
This book contains the shadowy Navigator Houses, astropaths, Adeptus Custodes, A Thousand Son adept, World Eaters, a rock-hard Death Guard, a Luna Wolf and an Emperor's Children marine. Most of the characterizations are well done, and the Thousand Son is very well fleshed out, with motives and a raīson d'etre - unfortunately, the rest of his band are not quite so well done and in my opinion lack a real reason for being there.
There are scenes which are very well done, and which give great insight into the characters motives - such as the Hunter, who is now on my top 10 list of great HH (non-marine) characters - but also scenes which are there for no discernable reason - such as the Vacant Angel story-line, which I just found annoying and "Deus Ex Machina" in it's implementation. It could have been better done. As for things which could have been done better, there are the repeated show downs between armoured and armed Custodes and unarmed World Eaters, which I found my suspension of disbelief not quite capable of handling. After reading about the prowess of the Custodes in "The First Heretic" by ADB and "A Thousand Sons", it is my opinion that the Custodes would be dealing out far, far more hurt than is the case here.
This book is what a comrade of mine described as a "filler book" - and he is not wrong. It is entertaining (I enjoyed it), but it is not "great" in the Homeric epos kind of way, and does not move the pace of the HH line forward in any discernable manner.
Read it if you have time on your hands, and want to know more about the background universe of the setting...
Regards,
JBP