This work is a lot more than a bestiary of assorted 'monsters' to throw at your Acolytes - although if that's what you are after you will find plenty combat-fodder for them. There's a lot about the nature of threat that they will face as they serve the Emperor, things that will widen your appreciation of the dark and grim nature of the setting and help you bring it to life in your shared alternate reality.
Divided into chapters based on monster type, there are in-character commentaries by Inquisitor Felroth Gelt of the Ordo Malleus - considered very knowledgable about the Calixus Sector but somewhat morally dubious: readers are advised caution - and each monster entry comes with suggestions and plot hooks to help the Game Master bring them into the plotline seamlessly. The final chapter takes this further, being a discourse on the use of adversaries, including scaling notes and additional rules that may be of use.
The first chapter, Mutation, begins with a discussion on what mutation actually is and how it is regarded by mainstream Imperium society - an outward form of inward corruption. Seems they've never heard of genetics! In places where they are not exterminated outright, most mutants face a life of dangerous drudgery as part of an underclass of serfs and indentured labourers. We then read about some notable mutant individuals as well as types of mutant that might be encountered, all with ideas of how they might turn up in your game to best effect.
Next, Chapter 2: Forbidden Science explores the use - or is that misuse? - of tech-science leading to the blasphemous experiments of tech-heresy. Some of these monsters are the results of such experiments, others are the experimenters themselves. Hideous fusions of man and machine are poised ready to rampage across your table, the saddest thing is that often the original experiments were actually attempts to improve the lot of humankind rather than to threaten it. This section in particular demonstrates the deeply-embedded religious nature of the game, that your Acolytes stand not just for 'truth' or 'justice' or 'fairness' but as bastions against the encroachment of evil against the light that is the God-Emperor.
This is followed by Chapter 3: Death Worlds. These are planets that are too dangerous to support much in the way of human settlement yet for all the danger they pose they are often the source of valuable resources. Dangers can be anything from the very environment to vicious wildlife and malevolent plants; and several well-developed death worlds are presented here to use directly or as inspiration for creating your own.
Then comes Chapter 4: Vermin and Predators. There's plenty of those around! There's a short discussion about how to use them to effect in your game, then a good ten of them are presented in gory detail ready to use. Each, of course, comes complete with descriptive text, illustration, stat block, plot hooks and Gelt's notes all aimed at empowering you to bring them to life as vivid threats to your Acolytes.
Moving on we come to Chapter 5: Xenos. This section provides seven well-detailed alien races as well as notes about utilising aliens in general and plot hooks for involving all seven (preferably not at the same time!). Whilst the Imperium views Xenos races as a threat, the feeling is often mutual, especially amongst those that once lived in the area now designated the Calixis Sector but who were driven out. An immense amount of detail is packed in here, enabling you to use these races with confidence.
The last 'monster' chapter, Chapter 6: Forces of Chaos explores the role of chaos and its agents as an ever-present dire threat to the Imperium, indeed to life itself. There are notes about how this threat can be brought to bear as well as eleven fully-described agents of chaos to bring into the game. Adventure seeds and realistic commentaries and notes accompany stat blocks, illustrations and descriptions.
Finally, Chapter 7: Adversaries discusses the core nature of Dark Heresy and indeed Warhammer 40K itself: unending war. But this is more than mere combat, dark and bloody: it's a story of survival and of hope, the hope of preserving the Imperium, imperfect though it may be, against the darkness that could so easily engulf it. Moreover, in running this game you don't just want brawl after brawl, you want to create and share stories that you and your players will remember long after the dice have been put away. Not all the foes they will face are as monsterous as the ones in this book, of course: many will be men just like them but with differing opinions and allegiences. There's discussion of balance, ways in which to face a few Acolytes with what appear to be overwhelming odds yet giving them a fighting chance. The use of fear of the unknown to effect, the involvement of the alien and the horrific - there are plenty of ideas for you to consider as you plan adventure or campaign. How to make things cinematic, how to let individuals shine, how to handle character death should it happen, and more for you to chew on as you actually run those adventures is to be found here too. Of all that I have read so far in the Dark Heresy range, this is perhaps the best bit about writing plots and running games that I have found so far and it will repay careful study.
Everyone needs a ready stable of monsters when running a game, and the final chapter alone makes this an essential work for every Game Master's shelf (or hard drive).