Since you tagged this NSFW, I can only assume that you’re looking for stories about sex.
I’m not a librarian but I have been in libraries in the past. I’ve seen college kids having sex under the table in the study room.
My building is very haunted. It has been featured on more than one ghost hunting show. Half of it is spooky and abandoned, I hear all sorts of odd noises from that wing all the time.
Someone used the library
After I graduated and moved away for my job, my high school had a senior jock who apparently liked hooking up with freshman girls in the library. According to the report, he liked it because the location forced the girls to stay quiet. It never said where in the library he was taking them but it did mention that it was the same location because of evidence being left behind. That evidence being some vaginal blood, spots of semen, and his used condoms which he threw on top of the shelving units; which is how he got caught. The janitorial staff were doing a deep clean of the library and found his collection.
I used to work in a University library. We had multiple incidents with frat kids bringing raw meat into the library, slapping it onto the table in front of unsuspecting girls, sometimes putting it between the pages of books. I’m not talking about little slices of bologna, either, I extracted a whole-ass steak from a tome in the science section.