Which Mage: The Awakening (and perhaps nWoD) sourcebooks do you consider essential for running a Mage campaign (besides the core rulebook, of course), and why?
Edit: Please refrain from recommending non-game books here. I'm sure many of us could list whole libraries of mainstream and obscure occult and esoteric literature here as possible inspiration for the game, but that is simply not what I'm asking for here. Thanks. :)