William G. Gray, better known to many as Bill Gray, was an English ceremonial magician, Hermetic Qabalist and writer, who published widely on the subject of western esotericism and the occult. Gray founded a magical order known as the Sangreal Sodality.
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This is an excellent, must read book on magic. In it Gray walks readers through a number of magical operations and shows you how to take those operations apart and work with them. I like the depth of theory he explores and find that his approach really makes magic easier to understand. I will note that his writing style does tend to be ponderous at times, but if you stick with it, you will get a lot from his work and be able to significantly improve your own magical practice.