Books about popular songs are primarily at these call numbers: ML1600-1651; ML2400-2551; ML2800-2881.
Some primary resources:
Christmas carols are in the library at M 2065 (accompanied) and M 2085 (unaccompanied).
You can use the library catalog to find a specific song. The best way is to search the song title as a phrase.
Popular songs are usually in collections at these call numbers:
Some primary song indexes:
Songs published singly are usually in sheet music format. Sources for sheet music:
Sheet Music Collection - Music & Dance Library
A title index to the library's sheet music collection is in the card catalog near the computer stations. Request songs at the service desk.