The Book of the Law

Liber AL vel Legis, commonly known as The Book of the Law, is the central sacred text of Thelema, allegedly written down from dictation mostly by Aleister Crowley, although his wife Rose Edith Crowley is also known to have written two phrases into the manuscript of the Book after its dictation. Crowley claimed it was dictated to him by a discarnate entity named Aiwass. However, the three chapters are largely written in the first person by the Thelemic deities Nuit, Hadit, and Ra-Hoor-Khuit respectively, rather than by Aiwass/Aiwaz.
Through the reception of the Book, Crowley proclaimed the arrival of a new stage in the spiritual evolution of humanity, to be known as the "Æon of Horus". The primary precept of this new aeon is the charge to "Do what thou wilt".
The book contains three chapters, each of which was alleged to be written down in one hour, beginning at noon, on 8 April, 9 April, and 10 April in Cairo, Egypt, in the year 1904. Crowley says that the author was an entity named Aiwass, whom he later referred to as his personal Holy Guardian Angel. Biographer Lawrence Sutin quotes private diaries that fit this story, and writes that "if ever Crowley uttered the truth of his relation to the Book," his public account accurately describes what he remembered on this point.
Crowley himself wrote "Certain very serious questions have arisen with regard to the method by which this Book was obtained. I do not refer to those doubts—real or pretended—which hostility engenders, for all such are dispelled by study of the text; no forger could have prepared so complex a set of numerical and literal puzzles[...]"
The book is often referred to simply as Liber AL, Liber Legis or just AL, though technically the latter two refer only to the manuscript.