For those who don't immediately recognize it, the Principia Discordia is the “founding document” of the generally-credited-as-satirical religion of Dicordianism, which worships the Greek Goddess of Discord, Eris. This volume is 1/3rd very interesting introduction (which I'm not sure is in other editions) and 2/3rds the main document. According to the introduction, Thornley and Gregory Hill (aka Malaclypse The Younger, credited as author), “in 1958 or 1959 in a bowling alley in Friendly Hills or maybe Santa Fe Springs, California”, had an experience with Eris, which resulted in Her taking up residence in Thornley's pineal gland, thus generating all this fascinating text.
The edition at hand is (once out of the introduction) quite graphic, in a “pasted-on-a-lamppost broadsheet by unstable persons” kind of way, with each of the “canonical” 75 pages numbered with a numbering stamp (as 00075, etc) and generally tarted up with assorted random rubberstamps and purloined clipart. The official title is “Principia Discordia, or How I Found Goddess and What I Did To Her When I Found Her – Wherein is Explained Absolutely Everything Worth Knowing About Absolutely Anything”, this being “The Magnum Opiate” of Malaclypse the Younger. The text meanders through various conspiracies, cosmologies, psychologies, and assorted theories about, well, absolutely anything, spinning out a strange (if somewhat incoherent) web of all things Erisian. The most familiar element here may be the symbol, “The Sacred Chao” which is a Yin-Yang symbol on its side with Eris' golden apple (inscribed with “Kallisti”) on one side and a pentagon (symbolizing order) on the other, expressing how order breeds chaos and out of chaos … well, there are theories.
For something so odd, Discordianism has had quite an effect in the culture. It claims The Church of Sub-Genius (of Bob Dobbs fame) as a sister religion, with Discordianism providing credentials as a “Pope” and Sub-Genius providing credentials as a “Tsar”, with other obvious similarities. Because the Bavarian Illuminati “are totally infiltrated” into the Discordian ranks, Thornley also claims the cult of Eris to have been the inspiration for Wilson & Shea's Illuminatus Trilogy, and subsequent memetic ripples through the cultural unconsciousness. Of course Discordianism had its own inspirations, such as the strange figure of Emperor Norton, whose exploits get a certain amount of attention in the book (perhaps as a figure of "Discordian governance"?), but it would be impolite to the Goddess to imply that all this material did not emanate from her specific beneficence.
As noted, this copy of the Principia Discordia is a long-gone and out-of-print edition (although copies are available via Amazon's new/used vendors, some with ridiculously high price tags), but other, more recent, versions do seem to be available, although if you're looking for the 60's graphics, it appears that you're going to have to go with used. On-line versions (such as http://www.principiadiscordia.com/) are similar, but most seem to have been re-set in computer faces (or to HTML text), and so have lost a good deal of the retro charm.