Entering the stacks of the New York Public Library requires a risk-benefit analysis. The benefit is that you'll find the book you're looking for.
The risk is that the lights will fail, and the many-eyed ones will creep out from the shadows. Their limbs are long, and propel their writhing bodies speedily through the stacks; their mouths gnash with sharp teeth.
I asked the head librarian why the library tolerated the presence of these creatures. Why allow patrons to be devoured? Surely that couldn't be good for circulation. He fixed me with a sharp glance and said, "They're extremely useful. We can spare an intern now and then."
"Useful for what?" I said. "Eating silverfish?"
"And thieves." He leaned in close and whispered to me. "I'll let you in on a little secret, little shaman. Oh, yes, you can't hide that smell from me. You're going to speak to spirits and gods and eat mushrooms and do wild dances, I'll be bound, but there's other types of magic running around this city, and this library is sitting on top of a pretty important source." He glanced around, then said, "The sub-basement decks are where we keep the books of wizardry."
"What does that have to do with the creatures?"
"Everything. Where do you think they come from? Those monsters aren't part of this city. They don't belong here. Wizard Magic is about drawing power from Elsewhere. And it has its unintended effects. You see what it does to the shadows -- it turns them into Things. Which, as I said, are very useful against the sort of people who would break in and steal important rare books, maps, et cetera."
"But why did the library agree to keep the books here, without knowing what effects they might have?"
"NYU has a Wizarding department, and the university gave us a large donation to keep the books somewhere most of the public wouldn't have potential access to them. Who could pass that kind of money up?"