The central text of Thelema is The Book of the Law. This text was transcribed by Aleister Crowley on April 8, 9, & 10, 1904 in Cairo, Egypt. Crowley “received” this text through the entity Aiwass. Interpretation of this book is considered a matter for the individual, and openly promoting personal ideas about its meaning is strongly discouraged.
You can read Liber AL for free here.