A very well-written examination of what it means to be an archmaster. The content is imaginative and epic in scope. It is not an essential supplement however; I have not once encountered a Mage game that necessitated busting out a rule book to sort out how archmasters work. If the topic interests you, you won't be disappointed. If however your Mage collection lacks certain essentials such as order books or Tome of the Mysteries, you would do better to invest in those as opposed to Imperial Mysteries.
Pros: Awesome dissertation on how to become an archmaster, as well as how archmasters interact with each other and a world that they can bend over backwards. Contains a chart that gives you a target number for affecting everyone on the planet.
Cons: Contains a chart that gives you a target number for affecting everyone on the planet. Awesome material of very limited utility in most games. Cover art is hideous; I mean, what the hell?
UPDATE 1/19/2012: My print-on-demand copy arrived last night. Once again, excellent quality.
[4 of 5 Stars!]