I've just reread Promethea. For those of you coming in late, this is a comic book written by Alan Moore and drawn by J H Williams III. What it isn't is like other Moore works, such as Watchmen or The League of Extraordinary Gentlemen; these can be enjoyed as stories without getting into the subtext, psychology and politics in the case of Watchmen, or playing spot the reference in League. That won't work in Promethea, or, at the very least, you'd be very puzzled by Issue #12.
Moore used the comic to introduce his ideas on magic, philosophy and religion to the world. It explains amongst other things what the Universe is, what God is and what magic is. It tells you the magical symbolism of the holy grail, how to navigate through the Tree of Life of the Kabbalah and what the Apocolypse is.
The art is gorgeous, and unusual designs are used to illustrate the subject matter ; Issue #12 for example is a frieze, which, in theory, could be a mobieus loop.
The Apocalypse is a weird thing indeed, and one which nearly disrupts the whole story. But Moore gets away with it. So there you are: everything you ever wanted to know about what goes on in Alan Moore's head, in 32 issues.
(You want more? Try Alan Moore on children's TV and Warren Ellis on Alan Moore on the phone (the link towards the bottom), both from the indispensable BeaucoupKevin)
 In which the history of the human race (and, indeed, the universe) is explained in terms of the 22 major arcana and in which the major arcana are explained in terms of the history of the universe. By two snakes on Promethea's caduceus.
 The manifestation of God's love in the Universe.