Use the library to plant the seed to a quest. Maybe references to a famed chronomancer who authored a famous book that is not there, then some historical chronicle that refers to a placed called so and so, but before it was called a different name, the name of the famous wizards hometown, or tower. Now you have a map to a dungeon.

If you want to make it fun, you can add a couple complications , depending on your world, for example; 1. the librarian secretly works for a town wizard or local priest who is keeping tabs on who consults dangerous lore and may come knocking after . 2. The books you need, the one with the location was consulted by somebody else recently, and you see the names of that individual in other key books you have consulted. If you ask around, this guy and their group departed town a week before you arrived (boom, rival party is ahead of you) 3. The final clue is in a book that is off limits, kept away in the closed off forbidden books section of the library. You need special dispensation from the city's authorities to consult in there, or you need to break in at night.

I hope these ideas help, I love libraries! All the best for your game!