You can create an air vent by drilling four small holes in the floor of the Library. Drill the holes close to the front (near the door) so that the moisture can flow out without reaching your books.
Also consider placing a dish mat or another inexpensive item on the floor of your Library. That will lift the books slightly off the floor and allow air to circulate beneath them, and it will also protect those books in case some water seeps inside when the door is opened.
Some stewards place moisture absorbers like DampRid inside their Libraries. DampRid pulls moisture out of the air and may reduce the humidity inside your Library. If this interests you, stop by your local hardware store and ask for their recommendation.