I took this picture in 2011, but with a little tweaking it looks like it could have been taken a century ago. Let’s pretend it was. If our olde tyme camera could have captured some people in this library, who would they have been? Old white-haired men smoking pipes? Serious-faced boys with slick-backed hair? Young women with high-collared blouses and Gibson tucks? What would they be wearing? Jackets and ties? Robes and regalia?
Where is this library located? In a city? In a private collection housed in a castle in Germany? In Washington, DC? Is it open to the public? How would your character get inside?
As libraries go, it looks fairly bright and cheerful. But what does this place look like after dark? Would it be a good place to study late at night? Have a midnight tryst in the stacks? Murder somebody?
Is something hidden within the library? Underneath it? Does the librarian have a secret that keeps him working late into the night, long after the last patron has gone home? What do the janitors do there at night?
What kind of books does it house and what do they keep in the restricted section? Could the knowledge in one of these books save you? Kill you? Change the world? Would your character know how to find one of those books using the card catalog if s/he had to?