Essential for anyone seriously interested in 20th century (and beyond) occultism, Western Mystery tradition, and Western esoteric work. I consider it the "Old Testament" of the above, and although I've had it on my shelf for over 20 years, I still revisit it on occasion. It contains commentary and ideas touching on pretty much everything related to Eastern/Western metaphysics, spirituality, yoga, occultism etc. which was known (in the West) at the turn of the 20th century. Not all is correct (especially AC's take on Eastern philosophy, which is purely his own limited understanding and subjective experience- and which would change later on), and the wordiness and intellectual style of writing that Crowley uses makes it hard to penetrate for most beginners or less intellectual types. Still, there are some hidden gems scattered throughout, practical instructions/rituals for starting on the AA path, fundamental texts related to Thelema (Liber AL etc.), Qabalah correspondences tables, a good summary on (Raja) yoga, and some great conceptual diagrams.
As a consolation for those who find it complete babble, or unintelligent garbage- this heft volume will still look impressive on any bookshelf (and form a very effective paper weight or door stop).