Signs of Sorcery is the latest supplement for 2nd Edition Mage: the Awakening. It bills itself as a deeper look at Supernal magic, but since more or less all the magic in Mage is Supernal in nature it’s essentially a big set of both advanced theory and further clarification on all sorts of aspects of the cosmology of the game. This includes discussions of Mage Sight, the Supernal Realms, various types of magic item, the sort of traces left behind by the Archmages, the Exarchs and other major powers of the Supernal World, Yantras, Grimoires, and many other topics besides.
This is a great help in many respects, not least because Mage: the Awakening is a classic example of how brevity is not always clarity. Sometimes a brief explanation of a subject will actually be more confusing than a longer, more in-depth discussion of it. This is particularly the case if the in-depth discussion reveals more of the rationale of why a particular thing is the way it is; if you have an appreciation of the underlying logic of a subject, then its surface facts will be that much easier to recall than if you’re just presented them as a set of disconnected facts with no underlying information typing them together.
Of course, if your Mage campaign is ongoing you might have already reached rulings on a lot of these subjects that are incompatible with Signs of Sorcery‘s take on them – but even then, it’s still a handy resource to have for any campaign where poking the metaphysic is a major part of the agenda, and given Mage‘s themes that will likely account for a high proportion of Awakening campaigns.