A collection of Aleister Crowley writings ranging from densely metaphoric poetry to 'Liber Legis' (Book of the Law). This is the foundational text of Thelema. It defines the start of the Aeon of Horus, which is described as the next stage of human spiritual growth. Additionally, the Thelemic Law: "Do what thou wilt shall be the whole of law" is put forth as the singular commandment.
Other important Thelemic concepts are discussed, usually through metaphor and allegory. In particular, the idea that each individual must determine their own interpretation of their 'True Will'. The lack of well defined dogma and normal religious tenets such as an afterlife, taboos, etc is explained as being in keeping with the law. This is an important concept, as the tendency for religions to become dogmatic, while separating the average practitioner from first hand experience, is seen as one of religion's largest faults by Crowley and company.
The introduction, along with Liber LXI, was particularly important for this book as it provided a lot of important historical information concerning the creation of A∴A∴. Crowley, normally referred to as Frater Perdurabo or Frater P., had been a member of the Golden Dawn Order. He and the leader of that order had a falling out that lead Crowley to go to Egypt where Aiwass, Crowley's HGA, dictated Liber Legis to him over the course of three days. Upon returning to England Crowley began the A∴A∴ using the lessons learned in the Golden Dawn. This is why each grade in the A∴A∴ has tests (Apparently in the GD promotion to higher grades was more of a good ole boys thing), why each member in theory only interacts with his direct superior and subordinate and so forth.
The appendices also contain some interesting information. A description of the different grades of texts is described. Liber Legis is described as a 'class A' publication, meaning it's source is divine/praeterhuman/mystical. These texts cannot be changed in any way, as supposedly their style, lettering, punctuation, etc are meaningful and themselves contain hidden messages. Class B publications are scholarly works, class C are texts deemed to be suggestive or important, D are official ritual instructions and Class E are public statements, manifestos, etc.
Overall, an important book for anyone interested in A∴A∴ and/or the occult.