In the James Lee Burke novel "Jolie Blon's Bounce," he introduces a character named Legion Guidry, a man who appears by the end of the book to be, if not the Devil himself, then at least pretty high up in Lucifer's chain of command. Burke has flirted with the supernatural before, with the ghosts of "In the Electric Mist with Confederate Dead" and the dead partner who occasionally shows up to offer advice to Billy Bob Holland in "Bitterroot" and "In the Moon of Red Ponies."
I mention this because I love this sort of thing in a mystery -- just a hint of the paranormal, an element of mystery that might persist even after the main plot is resolved. Recently I read Ruth Rendell's "13 Steps Down" and it initially seemed that she was hinting at the actual ghost of a serial killer making his presence known to a young man with an unhealthy interest in such things. In the Karin Fossum book I just finished, "He Who Fears the Wolf," there were a couple of occasions where it seemed that one bizarre character might indeed possess paranormal powers. I found the books better because of these things, and confess to a slight disappointment when all was explained in a conventional way.
What other crime fiction has incorporated supernatural elements in a subtle, sophisticated way? Is this something that draws you in, or puts you off?