How much of the Illuminatus! Trilogy is fact, and how much is fiction?

In Robert Anton Wilson and Robert Shea's Illuminatus! Trilogy I see countless references to historical themes of Rosicrucianism, magic, gematria, kabalah, astrology, alchemy, and just about any other occult theme you could think of... but although I've read through the trilogy two or three times now, I must admit I have a pretty weak background in the subject and I am having a hard time finding where to go next. I've got a number of books on Aleister Crowley, astrology, tarot, kabalah, the Golden Dawn, and even Nazi occultism. Illuminatus! is, however, a more engaging read for me and although I appreciate it for being a fictional account of a real-life phenomenon, I wonder in reality how many of the characters, historical persons, events, and philosophies actually have any bearing on the real mystery schools or even the real Illuminati (if they indeed exist under that name, anymore?) For example, who is Hagbard Celine? And is there really such thing as a Principia Discordia?