Adam Gorightly has put up a meaty, interesting post at Historia Discordia keyed to Week Five of the online Illuminatus! discussion that I've been hosting here. You'll want to read all of it, but here's a bit I especially liked:
In a previous post, I commented how Illuminatus! is a mental exercise of sorts in trying to distinguish what is true and what is false in the book. In their quest to bust the Illuminati, Goodman and Muldoon arrive at the theory that the clues they’ve uncovered suggest the same thing: that the reality of the Illuminati is both true and false. And perhaps that’s the final secret of Illuminatus! (maybe): that it’s partly true and partly false and it’s ultimately up to the reader to decide for themselves which parts are true and which are false—so it becomes a different reality tunnel for each reader/experiencer.
I've archived Adam's post on the links page at upper right.