Knowledge is a funny thing, especially in the 41st century. Much has been lost completely, other bits are deemed too dangerous to know... and a lot is kept firmly in the grip of the Adeptus Mechanicus, who regard technology in a sacred light as they worship the God-Emperor as the Omnissiah, the Perfect Sum of All Knowledge. Not for nothing are they called the Cult of the Machine God. Within the Calixis Sector, they are concentrated on the Lathe Worlds, and this book serves to convey a wealth of information about both the Adeptus Mechanicus and the three worlds on which they congregate.
Chapter 1: The Cult Mechanicus starts off by presenting their origins and history, with special emphasis on their role within the Calixis Sector. Going back to the earliest times it seems that Mars, in the Earth system, saw their very beginning and that it was here they established their first forges - for they control all production facilities for virtually all technology especially that of a warlike nature, and claim ownership of every discovery, be it new research or the finding of long-lost technological items from the distant past. It is said that they were established on Mars even before the God-Emperor came to prominence on Earth and were amongst the first to recognise his importance, if not divinity. Their hierarchy and organisation are covered before a discussion of their quest for knowledge and and their holdings in the Calixis Sector. Ritual is integral to everything that they do, and quite a good overview is given of that - as it underpins virtually all use of technology this is well worth understanding. Like any organisation, there are factions and sects amongst them, and some heresy as well.
Next, Chapter 2: Servants of the Omnissiah provides all the game mechanics necessary to creat characters (and, of course, NPCs) who are members of the Adeptus Mechanicus - a goodly array of new options, backgrounds, career ranks and more. This will enable you to model adherents of the different factions and bring much more depth to Adeptus Mechanicus characters wherever they are encountered in your game.
Then Chapter 3: Dominions of the Lathes looks at the Lathes system, reviewing the planets, artificial stations and indeed the vast array of holdings that the Adeptus Mechanicus has here. There's a lot of information in this section, and it will enable this area of space to come to life whenever the party happens to visit. There are even more sects and factions specific to the Lathes system for those who enjoy intrigue, and the whole chapter is liberally supplied with adventure seeds... not to mention it spawns quite a few just as you read through the information herein!
Finally Chapter 4: The Light of Reason is an adventure that will send Acolytes to investigate psychic disturbances in the Lathe System... who is dabbling in what dangerous areas and can they be stopped in time? They will have to pick their way through faction politics, explore the darker corners of the Lathes system, and perhaps may come to understand why the Adeptus Mechanicus thinks the way it does. There's plenty of support for the GM, from environmental hazards to atmospheric descriptions, and whilst linear in concept the adventure is constructed in such a way that the party will feel like they can find their own way through it to the ultimate climax.
With technology and the religious approach to it being so embedded in the Imperium, this is a useful book to have for general background: if you want the Adeptus Mechanicus to feature large in your adventures, you'll need it.