I have just finished All Things Are Lights by Robert J. Shea, and I enjoyed it very much and recommend it to anyone who enjoyed ILLUMINATUS! Although superficially it is a straightforward historical novel with plenty of action and romance, there is rather more material than I expected about secret societies and secret occult teachings. The Templars and Cathars feature prominently in the book, and Gnosticism, paganism, sexual tantra and the Assassins also are referenced. The book's hero, Roland de Vency, has a skeptical attitude toward authority and an agnostic attitude toward religions. The Wikipedia article about Shea says that his novels include "a few sly hints about the subjects of ILLUMINATUS!" but in connection with All Things Are Lights, this seems like a strong understatement.
While this blog focuses on Robert Anton Wilson, I feel strongly that Robert Shea also should not be neglected or forgotten. Kindle editions of Shea's works may be purchased from Amazon. Free copies of many of his books also are available from the official site maintained by his son, Mike Shea, and from a collection of some of his work at Project Gutenberg.