Sheet music can be an excellent source for images and lyrics relating to popular culture.
The Duke University Library has compiled an excellent guide to many collections of historic sheet music; the Chicago Jazz Archive at the University of Chicago Library has compiled a similar list. The following are links to some notable digital collections:
Historic American Sheet Music (Duke University Library)
E. Azalia Hackley Collection (Detroit Public Library; African-American sheet music)
Templeton Sheet Music Collection (Mississippi State University Libraries)
University of Oregon historic Sheet Music Collection (University of Oregon Library)
Iowa Digital Library Historic Sheet Music Collection (University of Iowa Libraries)
19th Century California Sheet Music (Mary Kay Duggan, UC-Berkeley)