I remember working late in the stacks when I was in college and going to the bathroom, opening the stall door and seeing a rat. I told the person at the desk who just said, “yup, we have rats back here.” End of story.
If I were the librarian, I wouldn't be so tolerant of rats. Besides posing an obvious health hazard to patrons and employees, rats seem to find book bindings to be tasty, probably because of the glue. A library cat would be advisable.