"The Tractate Middoth"
from Ghost Stories of an Antiquary, Part 2, by M. R. James
There are some majorly convenient coincidences in this one. It's quite a bit weaker than the two first stories, but there's a disturbing image or two to give you something to avoid thinking about.
The "eerily empty library" setting reminded me of the upper stories of "my" old university library, back before they renovated it. It was so quiet up there, and often there seemed to be no-one there. Maybe a few people using the computers or tucked quietly into one of the study nooks-- but when you were wandering the maze of tall bookcases in the dim light... ~shiver~ That place was creepy enough even without unsettling memories of ghost stories.