Babylon is referred to in several places in the Book of Revelation in the Bible (which may have had an influence on Thelema, as Aleister Crowley says he read it as a child and imagined himself as the Beast). She is described in Chapter 17:3-6:
So he carried me away in the spirit into the wilderness: and I saw a woman sit upon a scarlet coloured beast, full of names of blasphemy, having seven heads and ten horns. And the woman was arrayed in purple and scarlet colour, and decked with gold and precious stones and pearls, having a golden cup in her hand full of abominations and filthiness of her fornication: And upon her forehead was a name written, MYSTERY, BABYLON THE GREAT, THE MOTHER OF HARLOTS AND ABOMINATIONS OF THE EARTH. And I saw the woman drunken with the blood of the saints, and with the blood of the martyrs of Jesus: and when I saw her, I wondered with great admiration.
Aleister Crowley recorded a revelation of his own in The Vision and the Voice.
All I get is that the Apocalypse was the recension of a dozen or so totally disconnected allegories, that were pieced together, and ruthlessly planed down to make them into a connected account; and that recension was re-written and edited in the interests of Christianity, because people were complaining that Christianity could show no true spiritual knowledge, or any food for the best minds: nothing but miracles, which only deceived the most ignorant, and Theology, which only suited pedants. So a man got hold of this recension, and turned it Christian, and imitated the style of John. And this explains why the end of the world does not happen every few years, as advertised.
Also try this free pdf e-books:
Aleister Crowley - The Book Of The LawMichael Ford - The Book Of Cain