I wouldn't call this an expansion, because that usually means one can go without buying. No, for me this book is mandatory if you purchase Elements of Magic (revised). And you know what, I am completely okay with that, as this book is amazing. The information within (the new classes, the ritual rules, and the many many traditions and their feats [such as Scribe of Worlds, love it]) make this book necessary and very much useful. There is so much in this product that there are no words to describe how very cool it is. My only wish was that it was in physical form, with its sibling, so I could read it anywhere.
[5 of 5 Stars!]