Michael Johnson has a particularly good post on books up that starts out with a recommendation to read Fritz Leiber's Our Lady of Darkness and ends up discussing Illuminatus! (among other books.) He suggests reading the Leiber for Halloween. I wish I had read his post a few days ago; I've already begun reading Those Who Hunt the Night by Barbara Hambly for my Halloween read (vampires by gaslight in England -- it's pretty good so far.)
Leiber was a prolific writer who authored a wide variety of science fiction and fantasy, although as it happens, I still haven't gotten around to reading his most famous books. But here's a footnote for all of you RAW fans: He once reviewed Illuminatus! -- and panned it. It was in the February 1977 issue of Fantastic Science Fiction, a second-tier SF magazine edited by Ted White. (Now, I loved White's second-tier magazines, Amazing and Fantastic, but they didn't seem to have a huge number of readers.) I don't have the review, but I seem to remember that Leiber claimed Wilson and Shea made too many mistakes. One of the examples he cited was the spelling of "Fernando Poo" (he thought it should have been "Po.") If I can get my hands on a copy of the review, I will post it. Does anyone happen to have a copy?