This rather perplexing (at first glance) work has become a gem I've referred to, and found useful, in the oddest of times.
It's a reference book far more than a reading book (in the same way as a dictionary - you don't read it cover to cover) but the breadth of information it contains is fascinating. The "meat" of the book, as a previous reviewer put it, is an extended table taking as its starting point the numbers O to 32, each of which relate to a path or sphere on the Tree of Life. Each column takes a particular type of thing (a plant, a mineral, gods of various pantheons, or something much more arcane like the later columns - "the Angelic Functions in the World of Yetzirah" probably won't mean much unless you're pretty hardcore!) and slots it into which number / path it best fits under.
For me this has been invaluable in casting new light on the "character" of the paths / spheres, as I have been able to look at the attributions and work backwards to surmise about the path itself. I could have done with more explanation of WHY a particular correspondence has been made: although there are endnotes, it is plain they were bare essentials only. Many columns are written in Hebrew, which is annoying - perhaps if I can't read it it's because I don't know enough to use that column anyway, but you take my point.
The opening essay on Gematria is not only very readable and clear, but written with Crowley's trademark wit and chatty tone, which helped my understanding a mile.
This isn't necessarily a book just for Thelemites (I'm not one), as most occult-minded people will find themselves dipping into it endlessly, especially if you're into talismans (eg, Eclectic Wiccan). But it also isn't a beginner's book, as you'll definitely need some "background" in which to make sense of all this raw data.