The Tree of Life is a tool used to categorize and organize various mystical concepts. At its most simple level it is composed of ten spheres, or emanations, called sephiroth (sing. "sephira") which are connected by twenty-two paths. The sephiroth are represented by the planets and the paths by the characters of the Hebrew alphabet, which are subdivided by the five elements, the seven classical planets, and the twelve signs of the Zodiac.
Within the western magical tradition, the Tree is used as a kind of conceptual filing cabinet. Each sephira and path is assigned various ideas, such as gods, cards of the Tarot, astrological planets and signs, elements, etc. Within Thelema, the seminal book which defines all these correspondences is Crowley's Liber 777, although there have been other influential writers on the topic, including Israel Regardie and Eliphas Levi.
The path of attainment is largely defined by the Tree of Life. The aspirant begins in Malkuth, which is the everyday material world of phenomena, with the ultimate goal being at Kether, the sphere of Unity with the All. Through various exercises and practices, he or she attains certain spiritual and mental states that are characterized by the various sephiroth that ascend the Tree. Crowley considered a deep understanding of the Qabalah to be essential to the Thelemite:
The Tree of Life has got to be learnt by heart; you must know it backwards, forwards, sideways, and upside down; it must become the automatic background of all your thinking. You must keep on hanging everything that comes your way upon its proper bough.
and,
The art of using it consists principally in referring all our ideas to it, discovering thus the common nature of certain things and the essential differences between others, so that ultimately one obtains a simple view of the incalculably vast complexity of the Universe.
The whole subject must be studied in the Book 777, and the main attributions committed to memory: then when by constant use the system is at last understood — as opposed to being merely memorised — the student will find fresh light break in on him at every turn as he continues to measure every item of new knowledge that he attains by this Standard. For to him the Universe will then begin to appear as a coherent and a necessary Whole.
Part of the reason why the Qabalah is so important is that it is the key to understanding the Holy Books. Most of them, including The Book of the Law, are written in abstract, poetic, and often obscure language. Through the use of the Qabalah, and especially the function of gematria (a form of numerology), the normally opaque meaning of the texts can be made clear. Thelemites can also make use of gematria to link words and concepts and to validate revelations given to them in magical operations, such as astral travel.
Also try this free pdf e-books:
Austin Osman Spare - The Focus Of LifeAleister Crowley - Liber 913 The Treasurehouse Of Images
Belle Wagner - Within The Temple Of Isis