I really enjoyed this, but I would not call it a horror novel. It has very minor horror elements, but most of the book is about these two misfits over the years - I'd shelve it as just general fiction.

It's a pretty slow, melancholy "memoir" where Mercy finds Art's manuscript and writes in her thoughts and corrections to Art's narrative. I found the annotations fun - you don't know who is right or telling the truth when they contradict each other. He slowly starts to suspect there's something supernatural going on or that she might be a vampire, but she thinks that's ridiculous! It was admittedly slow in places but overall I had a great time with it.