Ash

It starts off as an historical romance set in 1466/77 full of rich detail of the medieval experience. A young girl growing up in a mercenary camp, raped, abused but still fighting to survive until she has a mystical experience that leads you think this might be fantasy after all.

But then you switch to a comtemporary timetrack with letters and emails about what has just been translated and you start to think this is the Fantasy version of A. S. Byatt's Possession.
And then it's back to the main document and you get golems made out of granite (huh?) and stone computers and a Visigoth civilization that inhabits ancient Carthage and is threatening all of Western Civilization and you realize you are in the wildest most detailed alternate history full of quantum mechanical theories of consciousness and whatnot that you are ever likely to read.

Even though it is enormous (1100 pages) it never flags. This nine month biography of one of SF's strongest heroines is a masterpiece of the genre.