Some web archives can be useful in accessing full-text primary resources in other library collections. You may wish to try the following sites to find, for example, 19th century magazines and books, including sources for American popular music. Some of these collections also provide access to online music recordings.
Google Book Search- full text books limited to only those published before 1923 (out of copyright) as well as "preview" content of newer books.
Internet Archive- a warehouse of many different kinds of media, but their e-book collection has many titles of use, again, primarily for those materials that are out of copyright.
Sheet Music Consortium - A project hosted by several academic libraries to provide sheet music collections from the 19th and 20th century. Lyrics and sound recordings are available for several collections.
DRAM Collections - A resource for online American music recordings of historical and cultural significance.
Smithsonian Global Sound - A robust archive of recorded music online from around the world. This collection includes resources for several American music genres.
Library of Congress, American Memory Project - The Performing Arts collections in American Memory provide a rich archive of resources on the history of American popular music over the last two centuries.