Guide to the Technocracy is one of the best Mage books published, and is essential reading for anyone looking to run Mage. The only way this will change is in the event of Technocracy: Reloaded being as good, but updated.
More specifically, this book is an in-depth guide to the world of Mage from the perspective of the Technocracy. It allows you to turn Mage from a Fantasy game into a Science-Fiction game, but still recognizably the same setting. It breaks down in more detail:
Prologue - The opening fiction isn't particularly strong, but it isn't bad. It shows an Amalgam taking out a Chantry, and the aftermath.
Introduction - The introduction sets the tone spectacularly, making it clear that (from the point of view of this book) the Technocracy are the misunderstood heroes, that the Traditions are the bad guys, and that the stereotype Technocrats of 1st Edition are not what the Union is. It hits the standard notes, and already begins adding depth, pointing out schisms within the Union and playing up a feeling of desperation for the Union...they may be winning, but Reality is still on the brink, and needs to be rescued.
Indoctrination - The first full chapter elaborates on the ideas in the Introduction: Reality is a mess, we need to save it, there are internal problems to go with the external ones, and that's why we need the player characters.
Enlightened Science -Chapter two is a deep dive into the Technocratic paradigm, or rather, the part of it that is consistent between Technocrats of various Conventions. It talks about the spheres, paradox and much of the Mage cosmology in these terms, explaining everything in scientific terms.
History Lessons - This chapter has a lot of meat in it: the Technocratic view of the history of the world, particularly the history of the Union. We see some of the schisms in action, following up on the Technocracy: New World Order book's discussion of interpretive lenses for history, though more concrete as is appropriate for what is functionally a core book (except for mechanics).
Protocols - Here's where the rules that Technocrats must live by are, such as the Precepts of Damian and the mechanics of Social Conditioning. It covers a lot of the day-to-day of being a Technocratic operative.
The Conventions - Five conventions, a bunch of methodologies, and a collection of examples fill out this chapter to help see the humans behind the technology and the masks of soulless uniformity that make the Technocracy so feared by the superstitionists.
Character Recruitment - Character creation, fairly standard. Includes rules for creating constructs, though they've largely been superseded, I believe.
Storytelling - This chapter covers how to run a Technocratic game, including what sorts of conflicts both internal and external are good for driving the drama, what sorts of resources the PCs get access to (by default), etc.
The Arsenal - BIG pile of Procedures and Devices.
Appendix - Recommended reading.