I couldn't stop once I started. Again, an AR with no magic, and I was halfway through before I realized. This had a whiff of Jane Eyre about it, with Snape as Jane, and Harry as the 'secret ghost' in the tower. I laughed at Snape's inner thought that his unnaturalness made him a freak, and I liked how you worked the other canon characters in, particularly Filch and Hermione. I was afraid that it was going to be too channish for me, but it didn't turn out that way at all.
Very nicely done; you certainly nailed your genres and prompt. Congrats.