Andrew Crawshaw has done a series of postings on one of his blogs, Drafts, which seek to provide a bibliography of ILLUMINATUS! A very interesting effort.
One of Mr. Crawshaw's entries has a footnote referring to Ishmael Reed as "aka Robert Anton Wilson." Kidding, I think, but it inspired me to look up Reed's Mumbo Jumbo on Wikipedia, and it sounds like an amazing novel and very possibly an influence on ILLUMINATUS! I will have to hunt up a copy.