Reference works include encyclopedias, thematic catalogs, indices, and guides to research. Many are printed editions that can be found in the reference section of the Fine & Performing Arts Collection. There are important electronic versions, as well, like Oxford Music Online.
A Guide to Research is a book-length bibliography of scholarship on a composer, genre, or special topic.
Thematic Catalogs are complete lists of works by a composer or in a genre. They often include incipits, the opening musical gesture of a piece.