If you are an Inquisitor, life gets pretty boring without a few heresies, cults and other such issues to investigate. This book goes a way towards alleviating the boredom, providing the Game Master with plenty of foul cults that hide, often in plain sight, throughout the Imperium emperilling the lives and very souls of honest citizens. There's a wealth of information (some of it even true) about what these cults do and believe.
The book is divided into several sections, each dealing with a different aspect of the problems your Acolytes may be sent to deal with. Heretics, mutants, psykers, xenos (aliens), and daemons can be found here, with all the legends and conspiracies that have grown up around them, liberally bestrewn with ideas about how to put them to use in your game. These first few chapters are followed by a chapter on the enemy within, looking at dissent, misunderstanding and strife in the very organisations designed to protect the Imperium. It's not all about weird beings and religious debate, of course, and there's a chapter jam-packed with crime lords and villains as well.
So, Chapter 1: Shadows of the Tyrant Star presents a collection of fragmented prophesies and legends about a rogue dark star that used to, or so it is said, float through the Calixis sector, dragging fear and terror and destruction in its wake. Much takes the form of reports and scrawled notes, suitable to present to your players - although you will have to mess around a bit to take them out of the distinctive page edgings if you are printing from the PDF, and if you own the 'dead tree' version, even a photocopy may not give as full a flavour as might be desired. It would be an enhancement to provide straight PDF versions for GMs to print out for use in play. Some suggestions are made as to how you might weave them into your plots or even base a whole campaign around this star.
Chapter 2: Hereticus then looks at the threats posed by heretics, mutants and psykers. Of course, in an Imperium ruled by an Emperor who is also a god, heresy can take the form of any kind of dissent - pursuing forbidden scientific research, say, political corruption or even being over-vocal in support of progress, it's not always a matter of having got your doctrine wrong or gone off worshipping other deities instead of dutifully following the state religion. Different kinds of cults are discussed, along with some of the rarer abilities some psykers manifest (mainly intended for antagonists but you may wish to allow more advanced Acolytes access to carefully-selected ones). A major heresy called the Temple Tendency is gone into in considerable detail ready for you to use as an underlying plot, background or even a major adversary, depending on your needs and wishes. And if that were not enough, there's another bunch called The Logicians. They seek progress through scientific method and rational analysis rather than religious faith. Again there's plenty of detail on them, as well as on the Pale Throng and the Night Cult, other mischief-makers with which to contend .
Then Chapter 3: Xenos explores the alien threat with, again, plenty of detail about what lurks in the blackest reaches of space and hungers to devour or corrupt the honest citizens it is the Acoloytes' duty to protect. More subtle than ravaging war-fleets, the threats are many and diverse, and there's plenty of scope for missions against them with the information provided here. This is followed by Chapter 4: Malleus, where chaos raises its ugly head to cast a malign influence where it may, the deadly interaction between humanity and the warp. Survival is impossible without it, yet the threat is immense and must be guarded against at all times. Sorcery, dark relics and more are to be found here, again with commentary about how they may be used to effect in your game.
Next, Chapter 5: The Enemy Within is all about dissent that could lead to open strife within the very power structures of the Imperium itself. Quis custodiet ipsos custodes? (Who watches the watchers?) Perhaps your Acolytes will be that bastion that stands firm. There's an overview of the causes of conflict, dispute, and factionalism within the authorities and powers that govern the Imperium, an opening of a lid on self-interest, territorialism and other blights that even those involved may not recognise as being against the interests of the greater good within the Imperium they honestly think that they serve.
Chapter 6: The Hunted presents the Most Wanted list, a fine array of transgressors for eager Acolytes to track down and bring to book, along with notes on villain design and the use of the nemesis. Plenty of good advice here for Game Masters to absorb as they plot away.
Finally, there's a scenario ready to play, pitting Acolytes against the dark forces that have gathered to misappropriate the legacy of a Rogue Trader. Called The House of Dust and Ash, there is plenty to keep your Acolytes busy, all presented clearly. Recommended for more experienced Acolytes of the 4th or 5th career rank, it comes in four main parts with plenty of optional encounters and room for expansion as you see fit. There's plenty of advice and scope for stamping your own mark on proceedings... and I'm wishing I hadn't read it now, it would have been fun to play!
Overall, this is an excellent resource, full of information to help you craft adventures that fit the ambience, the feel, of this setting. The Acolytes may not thank you for reading this, but their players will!