We offer the reader one of the most sublime and profound mystical texts that came from the pen of Aleister Crowley, an outstanding occult philosopher, magician and poet, founder of the religious and philosophical system known as “Thelema.” “Liber LXV”, or “The Book of the Serpent-Entwined Heart” is part of the corpus of the Sacred Books of Thelema - texts written down by Aleister Crowley under inspiration from above and forming the basis of his mystical teachings. It tells of the relationship between the Aspirant and his Holy Guardian Angel. In this edition, “The Book of the Serpent-Entwined Heart” is published with extensive commentary, the main part of which was written by Aleister Crowley himself. In the appendix for the convenience of readers who wish to refer to the text of this Holy Book directly, "Liber LXV" is played without commentary.