This is the first book I ever read specifically about a form of witchcraft, rather than having a small amount on that subject along with other matters, and it was a long time and a large amount of bookshelf space before I read a better.
The best thing about this book is that it is biased to a particular perspective and doesn't pretend otherwise. Ironically, the result is a much less biased book than many others on the topic: Crowley is writing as someone who is a High Priestess and also a Jungian and therefore does a good job of looking at Wicca from a Jungian perspective. While I personally think the Jungian point of view is over-represented in much Wiccan writing, and even more so in Neo-Wiccan writing, and often find that it grates on me (and when presented as the ONLY thing the gods are, irritates me greatly), this is not how I experience it with this book. Partly, this is because it doesn't pretend otherwise, unlike many books out there are there that treat Jungian archetypes as either undisputed fact or as if they were some sort of Wiccan dogma (which they aren't) or Neo-Wiccan dogma (which I suppose they quite possibly are). Partly also at least the author can actually write well about the Jungian perspective, rather than third-hand regurgitations of the idea of the gods as archetypes that doesn't really explore what that actually means.
The book also benefits from being focused largely on one way of looking at the Craft; in terms of the advantages it brings to it's practitioners (it's worth noting that the first edition was subtitled "The Old Religion in the New Age", and of course the New Age Movement differs from Wicca in being particularly concerned with what their practices can do for their practitioners' mental, physical and financial health). I'm not particularly interested in this view of what we do (for one thing, I think working the Craft solely for such benefits rather than in Their service would be a good way to lessen those incidental benefits) but for all that, I'm still much happier to read this book that tries to do one thing and does it well, than the libraries full of "the only book you'll ever need" that do so very little.
It's also why despite its wordier style than many books on witchcraft, this one has a lot more emotional resonance than most. It's rare to read a description of a ritual in a book on witchcraft, and to actually get any sense of why anyone would bother! Thankfully, this book is an exception to this.
I'd particularly recommend this to someone who had yet to read anything on witchcraft, and even more so to anyone who'd read about neo-Wicca and was interested in learning a bit about Wicca. I'd probably still put Gardner ahead in the list, but this would come in after that.