7x07 "The One with Ross's Library Book"

Ross is happy that his dissertation is in the school library until he finds out where it's located: the dead part of the library where people go to hook up. Rachel and Phoebe believe they've found the perfect girl (Kristin Davis) for Joey. Janice invites herself to Chandler and Monica's wedding, and they try to stop her by convincing her Chandler still has feelings for her. Ross, while trying to guard his dissertation from potential lovers, meets a woman who enjoyed his work, and they end up fooling around (and getting caught) in the library.