The Principia Discordia is a book explaining the beliefs of Discordians and their worship of Eris Discordia. It was allegedly written in the late 1950s by the first 2 acolytes of "Bob" Dobbs, who had been recruited back in 1953, but then followed the advice of "Bob", who told them "Start your own damn religion!", resulting in the first of many SubGenius schisms.
Some editions of it have an introduction by Robert Anton Wilson. Rev. Ivan Stang originally planned on suing the authors for plagiarizing content from the Book of the SubGenius, but then realized that Principia Discordia had been written several decades earlier and that The Book of the SubGenius had technically plagiarized Principia Discordia; evidently Discordians know about Time Control and used this to get their book published before the SubGenii. This means, in all likelihood, the Principia Discordia hasn't even been written yet, and will be written by people in the future and sent back in time, to reverse-plagiarize other books published in between.
Wikipedia tells more about this strange book from the future. Because it is from the future, it is not copyrighted by anyone, but is public domain and can be viewed for free on teh Internets legally on over 9000 different Discordian websites, in HTML format. Of course, there are also published versions you can buy, and many different publishers have published it over the years, since it isn't copyrighted at all.
Here you can watch the Principia Discordia as a video (you can pause on each page long enough to read it, then press play again to continue):