Sign of Four will definitely work.

I had made some vague reference to The Sign of Four in my prospectus because I hadn't finished reading it and wasn't exactly sure how the supernatural shows up.  While my mention of "Indian mysticism" being all over the place is not wrong, I now can add mysterious "ghosts" that turn out to be real people to the list.  When Holmes, Watson, Sholto, and Marsten go to see Sholto's twin brother about the treasure, he tells the story about the moment their father was about to confess the location of the treasure on his deathbed, only to be interrupted by a mysterious face appearing in the window.  This reminded me a lot of the scene from Wuthering Heights with Lockwood in Catherine's room, although Bronte makes it much more ambiguous as to whether or not that is actually a ghost.