The librarian gives Sheldon a book by behavioral psychologist B.F. Skinner. In The Gothowitz Deviation (2009), Sheldon uses Skinner's principles in his attempt to modify Penny's behavior.
At the beginning of the episode, Sheldon is watching the movie Conquest of the Planet of the Apes (1972). That movie was set in 1990, the same year in which this episode is set.