I plan to do a "Robert Shea Week," probably in late May, and I've invited people who read this blog to read his historical novel All Things Are Lights and discuss it with me when I post about the book that week. I plan to reread the book.
Here is Shea on the novel, from a 1985 interview by Neal Wilgus published in Science Fiction Review, a big SF fanzine in those days.
SFR: Could you describe All Things Are Lights for us?
SHEA: The title comes from a medieval philosopher, Scotus Erigena, who said, “All that are, are lights.” The main characters have an outlook that is as mystical as that statement, only their mysticism is not of the orthodox variety. The main character is a troubadour who achieves illumination in an adulterous affair with a countess through the rites of courtly love, which I portray as a Westernized version of tantric yoga. The troubadour is also in love with a woman minister of the heretical Cathar sect. Nowadays they tell women they can’t be priests; in those days they burned them at the stake for trying. These people get caught up in the disastrous Seventh Crusade led by King Louis IX, known today as St. Louis. The crusaders are eventually defeated by the Egyptian Mamalukes. The survivors, including the King, are held as hostages by the Moslems and try to save their lives by paying an enormous ransom.