Liber AL vel Legis, the Book of the Law, is three short chapters spoken in turn by three figures: Nuit, infinite space and the totality of possibility; Hadit, the infinitely small point of consciousness at the centre of each individual; and Ra-Hoor-Khuit, a form of Horus presiding over the present age. Crowley held that the text opened the Aeon of Horus, the crowned and conquering child, succeeding an Aeon of Osiris organised around a dying and rising god and, before that, an Aeon of Isis. The furniture is deliberately Egyptian; the content is not Egyptology, and Crowley did not claim it was.
The maxim everyone knows, "Do what thou wilt shall be the whole of the Law", is almost always quoted without its companion, "Love is the law, love under will", and without the qualification Crowley spent decades supplying. True Will is not preference and not appetite. It is the specific function a person exists to perform, which he compared to the orbit of a star, and discovering it is described as difficult, lengthy, and largely a matter of stripping away everything that is not it. Once found it is not optional. Deviation from one's True Will is the only real error in the system, and indulgence that obstructs it fails by the same standard as any conventional sin. Crowley also held that True Wills cannot genuinely collide, each being proper to its own orbit, which is his answer to the obvious objection that the maxim licenses harm.
The formula is not original to him. Rabelais put "Fay ce que vouldras" over the gate of the Abbey of Theleme in the sixteenth century, and the Hellfire Club took it up in the eighteenth. Crowley knew this and treated the earlier uses as anticipations rather than as sources.
In practice the goal of individual work is Knowledge and Conversation of the Holy Guardian Angel, a phrase taken from an older grimoire and understood as contact with one's own higher nature, from which the True Will becomes legible. Beyond that stands the crossing of the Abyss and the dissolution of the ego as a separate thing. The system is graded, demanding, and framed as an experimental discipline rather than a matter of belief; Crowley's standing instruction was to test the claims and keep a record.