The Equinox is a four-volume set of hard-bound publications released in the early 1900s by Aleister Crowley and associates. Allegedly the masterpiece of modern Hermetic and Thelemic magick, it contains some of Crowley's personal writings, many of the Holy Books of Thelema, various philosophical dissertations, certain Thelemic rituals appropriate for release to the general public, and other selected materials from Crowley's private collection.
Of particular interest in The Equinox is The Temple of Solomon the King, a lengthy and detailed Magical Record of Crowley's personal esoteric exploits on his journey across the Mongolian deserts. Some Thelemic magicians consider The Temple of Solomon the King alone to be worth the hefty price tag of the series, usually circa $200.
An online version of the first and third volumes of The Equinox is available here (http://www.the-equinox.org).