CMU Library Catalog
Oxford Music Online (includes Grove Music Online)
The interface for Oxford Music Online changed on December 7, 2017. Oxford no longer supports the search box widget I had set up within this box so you will need to go directly into the database by clicking on the link above.
Oxford Art Online
If your topic crosses over into art you may want to search Oxford Art Online as well. I noticed that 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.
Die Musik in Geschichte und Gegenwart (MGG) is a general encyclopedia of music. MGG is encyclopedic in the true sense of that term: it offers in-depth articles on every aspect of music as well as many related areas such as literature, philosophy, and visual arts. MGG Online contains the second print edition of MGG, published from 1994 to 2008, as well as current, continuous online updates and additions.
Citing Your Sources
RISM: The online catalog contains over 1,000,000 entries, mainly manuscripts from between the years 1600 and 1850.
Bibliographies & Handbooks
Browse the ML134 STACKS-4 section in Hunt Library
Try an Advanced Cameo 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)