The Library Key is a quest item from The Legend of Zelda: Oracle of Ages. When Link enters the Zora Village, he learns that King Zora is very sick. If Link brings him a Magic Potion, he will feel better again and grants Link permission to enter Jabu-Jabu's Belly, but only after Link cleans up the Zora Seas. He then gives Link the Library Key so he can research the problem in the nearby Eyeglass Isle Library.
Upon taking the Library Key to the library in the present, Link will be unable to open the Library, but if he uses his new-found Tune of Ages, he will appear next to the Library and its corresponding keyhole. Opening the Library in the past simultaneously opens it in the present, allowing Link to enter the Eyglass Isle Library.