Imperial Mysteries is perhaps one of the best supplements for Mage: the Awakening I have ever read. It makes for the perfect capstone supplement, revealing the nature of the higher states of existence, while keeping enough secrets and challenges to still make it playable. Archmasters are uniquely powerful but still incredibly human characters, making a campaign of this level something that is still compelling as opposed to just being the bully in a sandbox.
My only issue with the book is the fact that most of the artwork was reused from the Corebook or other existing supplements, but that's a minor nitpick. The cover art could have been better, especially after such impressive pieces for Seers of the Throne and The Tome of the Mysteries.
That said, I highly recommend this book to any Mage: the Awakening GM who is interested in the nature of Archmastery, and is looking to get a bigger picture to work with when it comes to the interaction of the Exarchs and Archmasters with the Fallen World. This book isn't just for playing being of this scale, but also offers advice on how they can be used as elements in a chronicle centered on non-archmasters in a way that doesn't feel like they're ST mouthpieces.
This is an excerpt from my full review on my blog. If you'd like to see the full article, kindly visit: http://wp.me/p5H0U-VM
[5 of 5 Stars!]