African American Music Reference * contains essays and liner notes related to blues, jazz, spirituals, civil rights songs, slave songs, minstrelsy, rhythm and blues, gospel, and other forms of black American musical expression. Includes books like the following selections (and many more):
Jazz Music Library * lets you listen to over 10,000 different streaming albums that spans many major and minor figures, like Thelonious Monk, Count Basie, and Billie Holiday, along with many different genres, like swing, hard-bop, soul jazz, etc. Also included with many (but not all) albums are extensive liner notes that can be excellent sources of information about that particular recording and where it fits into music history and culture; look for the "related documents" tab once you've opened an album to play.
American Music * lets you listen to almost 8,000 different streaming albums that span many different popular music genres throughout American music history. You can find music from genres like blues, gospel, and R&B. Like the previous resource, look for the "related documents" tab to find liner notes for more information about that album.