I can think of two.
The first one matches your description fairly well, so it's probably what you're looking for. It's "The Library Tease" by Cariny:
http://cmnf.coccozella.com/board/viewtopic.php?t=1635
My favourite library story is "Ashley and the Library" by Jappio, however, so I will also post a link to that:
http://cmnf.coccozella.com/board/viewtopic.php?t=1856