Four authors who specialize in secret histories—novels that mix the real history of the world with strange, sometimes supernatural conspiracies—have written a round-robin discussion of their genre for Eos Books. The participants are John Crowley, Jeffrey Ford, James Morrow, and Tim Powers; I've never read any books by Ford or Morrow, but Powers' stories are usually a lot of fun and Crowley is one of the best novelists working today.
Powers offers the most perceptive comment:
[C]onspiracy theorists want "an alternative to the usual story we have to live with all day every day" (and we're all honorary conspiracy theorists when we're in the middle of reading a book like, say, Pynchon's Crying of Lot 49). But it has to be secret, too! There has to be that element of "I know the real story, and most people don't." I'm a sucker for that effect myself, so I like to try to evoke it in my books.
I've found lots of historical characters whose lives could plausibly (well, almost plausibly) include familiarity with, and action in, very secret magical goings-on, and it's fun to concoct supernatural explanations that cover even their recorded, apparently-mundane actions. But of course it's cheating–as Dan Brown cheated in The Da Vinci Code–to falsify actual recorded history. You have to leave the reader thinking, "Jeez, I can't see anything that contradicts this theory…"
He also gets off the best line, in response to Ford's remark that he doesn't think a writer can escape all the assumptions of his own time:
One time I was going to write a book involving the Brontes, and I read a 1990-something biography of Emily Bronte, and she turned out to have been an anorexic proto-feminist lesbian–but I think any woman in history would have turned out, in a 1990-something biography, to have been those things.
Crowley gets off a good line too:
I once told an audience at a convention that my Aegypt books were entirely organized according to astrological principles designed to draw through the text certain planetary influences that would cause readers to experience the text in a certain way. I quickly recanted, after seeing their many upturned faces filled with awe and acceptance.