There are numerous books and other sources of information which may prove of use to the aspiring Thelemite.
Needless to say, the resources supplied and suggested here are by no means exhaustive; rather, we have chosen the most central and accessible works. Each individual must decide for himself or herself which of these are useful, and which further paths to explore.
Key Thelemic texts
The book The Holy Books of Thelema includes most of the books which Thelemites consider to be Crowley's "inspired" texts, and which form the canon of Thelemic Holy Scripture. The chief of these is Liber AL vel Legis, sub figura CCXX, commonly called The Book of the Law. The contents of this book are rather cryptic, and Crowley has prepared a number of commentaries for clarification. Most of these are included in Book 4. Thelemites are expected to interpret the book for themselves, based on Crowley's commentaries and other writings; but are discouraged from promoting their personal interpretations to others.
Another book which forms an important part of the Thelemic canon, but which is not included in The Holy Books of Thelema for technical reasons, is Liber XXX Ærum vel Sæculi, sub figura CDXVIII, commonly called The Vision and the Voice.
The I Ching and the Tarot (considered as a book of mystic illustrations rather than as a fortune-telling device), though of pre-Thelemic origin, are also considered to be part of the informal Thelemic canon.
Recommended introductory reading
Newcomers to Thelema and the OTO often ask what books they should read to familiarize themselves with these subjects, or to undertake a study of Magick. This is a complex question, not least because each seeker will find different books essential or useless. However, there are a few books which convey core aspects of our system which will be of value to almost anyone.
This book is not especially "introductory" in the sense of being easy to understand, but because it is the very foundation of Thelema, it is included on this list of introductory materials. Continual study will tend to lead to further insights. As mentioned above, Crowley and others have written commentaries and auxiliary study materials relating to this book, all of which may also be helpful, and some of which are included in Book 4 (see below).
The Holy Books of Thelema by Aleister Crowley
This book includes The Book of the Law in addition to all of the other Class A materials (Crowley's "inspired" works), and is therefore the complete foundation of Thelema.
The Magick of Aleister Crowley: A Handbook of the Rituals of Thelema by Lon Milo DuQuette
In one volume, this book provides the key texts and rituals needed to "get" Thelema, together with insightful commentary on Thelema, magick, and ritual. This is probably the most accessible work on Thelema for modern readers.
The Mystical Qabalah by Dion Fortune
Qabala, the ancient Hebrew esoteric philosophy of numbers, words, and their connections to God and the universe, forms one of the pillars of modern magical practice. Much of Crowley's work is based on Qabala, and thus understanding his works demands a thorough grounding in Qabala. In addition, many practitioners find that Qabala forms an ideal basis for their own studies and workings. This book focuses exclusively on the Sephiroth with a complete lack of attention to the paths on the Tree of Life, but it is an excellent introductory text on modern day qabalism.
Magick in Theory and Practice by Aleister Crowley and others
This book is currently out of print as a separate volume, but is included in Magick: Liber ABA: Book 4. This is the indispensable book for those who wish to practice Thelemic ritual magick. This book is more challenging to read than The Magick of Thelema but if you want it straight from the horse's mouth, this is the place to go.