An article in the Telegraph goes a little bit into Holy Blood, Holy Grail territory, dismissing some of the wildest stories about the Knights Templar but also establishing that some of the weird allegations against them were true. The Templars crop up in RAW's works and also play a major role in Robert Shea's All Things Are Lights, which I've argued is a kind of thematic prequel to Illuminatus!
(Via Greg Taylor, who probably wants me to mention that he has a new book out, Stop Worrying! There Probably Is an Afterlife. His website is worth a few looks, too. )