Boredom, Harry. Terrifying and apocalyptic boredom from months of being in a place with nothing at all that actually needs doing. That's how Bahorel figured it out. (He's considered that the Library does this sort of thing to him on purpose. If so, he appreciates the entertainment.)

He stops in front of a section of books that are especially brightly lit by the criss-crossing lights above. In the nearby shadowed parts of the shelves, the books are audibly rustling. "There's what we're here for-- some of it, anyway. The better books." If Harry wants to read the spines, there are a few with titles like "Vampyre" and "Living Dead: A Field Guide" and "The Monster Book of Monsters". And quite a few with no title at all.