Oxford Music Online (includes Grove Music Online)
Oxford Art Online
If your topic crosses over into art you may want to search Oxford Art Online as well. I noticed there is an article for John Cage in both Oxford Art and Oxford Music. The content differs enough that depending on your focus one might be better than the other for your topic. There is also an article on 'music and art' in Oxford Art but there isn't a comparable article in Oxford Music. There is an article in Oxford Music for Impressionism that could be useful if you're more interested in composers who contributed to that movement.
Music Index with Full Text
Citations and some full text of articles about classical and popular music, musicians, and the music industry for more than 475 periodicals, as well as book reviews, obituaries, news, and selective coverage for more than 230 periodicals. Covers 1970-present.
RILM Abstracts of Music Literature
A comprehensive bibliography on writings about music featuring citations, abstracts, and indexes. It covers over one million publications from around the world on traditional music, popular music, classical music, and related subjects. Coverage begins in the early 19th century and extends to the present.
RISM: The online catalog contains over 1,500,000 entries, mainly sources from between the years 1600 and 1850. They also have printed music up to 1945 (e.g. music by Florence Price and William Grant Still). They have manuscripts from all time periods and are adding more 19th and 20th century sources. You can also browse by country/nationality.
Bibliographies & Handbooks
Browse the ML134 STACKS-4 section in Hunt Library
Try an Advanced Search with the term bibliography as a Subject and the composer's name or topic (e.g. chamber music) in All Fields
History of Music
Browse these titles in the Arts Reference area (4th floor of Hunt Library)
Diamond, Harold J. Music Analyses: An Annotated Guide to the Literature
FA-REF4 ML128 .A7 D5 1991 (Reference Book, Hunt Library 4th flloor)
Hoek, D.J. Analyses of Nineteenth- and Twentieth-Century Music, 1940-2000
FA-REF4 ML113 .N695 2007 (Reference Book, Hunt Library 4th flloor)