Throughout history, some individuals have profoundly influenced metal music and culture without playing a single note. Dante Alighieri. John Milton. Friedrich Nietzsche. Snorri Sturluson and Brynjolfr Sveinsson, who preserved the manuscripts that are now known as Norse mythology. Richard Wagner, whose dramatic music paved the way for today's epic metal. And, of course, the Beast 666 himself, Aleister Crowley.
Modern Satanism and a lot of today's occultism and Paganism wouldn't exist without Crowley's philosophy of Thelema - a self-empowering, martial blend of spirituality and ritual that draws upon Egyptian, Kabbalistic, Nietzschean and Tantric influences. Conceived in 1904, the Law of Thelema - do what thou wilt - was a potent antidote to Victorian attitudes. Metal serves much of the same purpose today.
More importantly, I'm not sure what Behemoth would sing about if it weren't for Uncle Al. Even if lighting a bunch of candles and chanting at midnight is not your horn of mead, you've got to thank Crowley for contributing to Behemoth's existence.
Spiritual initiation is, roughly speaking, the process of becoming more aware of yourself, becoming more efficient and in control of your life through meditation and ritual. Whether you believe this works through religious or psychological means, it's a big part of many people's lives. Initiation in the Aeon of the Child discusses this in the context of Thelema, which posits that now that we're in the individual age (in Crowley's historical theory: Aeon of the Child, the previous ones were the Aeons of the Mother (matriarchal) and the Father (patriarchal)), we have a different formula of initiation. No longer must the spiritual seeker symbolically 'die' and then be reborn, like Christ or in some respects Buddha, to reach 'enlightenment'. And the 'enlightenment' reached by seekers in this new era is (according to Gunther), psychologically speaking, different from the old conception, since it recognizes both the darkness and the light. Gunther explains it much better than I can: if you are interested, go read the book!
Gunther explains a lot of Crowley's difficult technical writings in language that's clear to the new seeker. He draws upon the psychological theories of Jung to relate these Kabbalistic or alchemical ideas to modern thinking, and he discusses the lifelong process of initiation in a topic by topic manner.
While I do disagree with Gunther on some points, for example, his suggestion that the resurrection formula - which is also the shamanic formula, mind you- is completely obsolete for every seeker, spiritual books qua spiritual books are intensely personal with respect to their authors. They cannot reveal absolute, scientifically proven truth, but only one person's perspective on something that's difficult to quantify. Gunther certainly understands Crowley's often obscure writing style, and while he does infuse it with his own strain of thought, this book will no doubt help many a newbie.
For what it is, Initiation is infinitely better than a lot of spiritual texts you will find floating on the 'New Age' shelf, such as "Teen Witch".