This is honestly the second half of the Mage 1e Core book. It finishes almost every thought that that book had, and opens the doors to further elaboration, moving the game towards 2e and softening the black & white feel of the original core.

The lease interesting section is the first chapter, which is just a pile of new Abilities, Archetypes, Background and the introduction to Merits and Flaws (which had been circulating in the other lines by this point.) The merits and flaws are particularly fun because some of them seem radically incorrectly priced. I admit, I'm also down on the extra Abilities, because I'm anti-secondary abilities in general with a very, very small number of exceptions.

Beyond that, the book really starts delving into how Mage works and how to actually make it work in practice. Chapter 2 just goes faction by faction, interspersing fiction with a discussion of each one's philosophy. It also includes the first hints of sympathetic Technocrats in the line, which gets expanded on later in the book.

Chapter 3 is rules, and Mage honestly needed rules badly. Clarification of magick, introduction of Talismans, Do, etc. Certamen gets its first rule set (though that's something Mage still struggles with) and some rules for computers are brought in, which combine with the Virtual Adept Tradition Book and Digital Web to make Mage have the most thorough rules for 1993's computers in any RPG I've ever seen.

Chapter 4 is great for players, but not as interesting to me now that we're in 20th anniversary edition: a list of rotes and wonders (and some mundane equipment). It gives a roadmap for actually creating interesting effects in the game, though many need updating to be used at a table today.

The final two chapters really work best as one. Chapter 5 is really interesting, showing in-world parables for the difficulties of a mage's path, learning from a mentor, how Avatars and Essences actually work, among other things. The strongest bit though, is the brief history of the Technocracy. Though some of it has been retconned (especially by the Technocracy books and Guide to the Technocracy) it gives the point of view of the Union and really makes Technocratic characters look interesting. It closes out with the final chapter, a collection of essays on writing mage, running mage, and how to interpret things for mage.

Overall, an essential book for Mage first edition, and still a useful book today.