Everyday I Write the Book - Gilbert B. Rodman's and Norma Coates' ever-expanding bibliography of (mostly) academic works on rock and pop music. - http://www.comm.umn.edu/~grodman/biblio/biblio-front.html
Dancecult - User-contributed bibliography of scholarly sources pertaining to electronic dance music culture. - http://www.dancecult.net/
Philip Tagg - Includes many online texts by this semiotician and seminal figure in popular music studies, specifically semiotic analysis linking musical structure with other aspects of culture and society. Also includes articles on teaching and research methodologies. - http://www.tagg.org/