Encyclopedias, Dictionaries, Guides, etc.
Try these first...
Oxford Bibliographies (Oxford Bibliographies offers exclusive, authoritative research guides. Combining the best features of an annotated bibliography and a high-level encyclopedia)
Oxford Music Online (Includes: The Oxford Dictionary of Music, The Grove Music Online, The Oxford Companion to Music)
Oxford History of Western Music ONLINE, an amazing narrative of the evolution of Western classical music.
Harvard Dictionary of Music (via Credo Reference-provides definitions of musical terms)
Baker's Biographical Dictionary of Music (via Gale Virtual Reference Library)
Contemporary Musicians (via Gale Biography Reference Center)
Networked Digital Library of These and Dissertations
Search this database for international dissertations and theses. Worlcat is also a good option.
Emusicquest: Music In Print (scores)
The Faber Companion to 20th Century Popular Music (via Credo Reference)
The Garland Encyclopedia of World Music Online (via Alexander Street Press)
Harvard Biographical Dictionary of Music (via Credo Reference)
Hofmeister XIX (Free on the Web)
Index to printed music, music books & periodicals, illustrations, portraits & busts of composers published or produced between 1829-1900.
International Dictionary of Opera (via Gale Biography Reference Center)
The New Penguin Dictionary of Music (via Credo Reference)
The RIdIM database is designed to facilitate discovery of music iconography images by registered researchers, and the description of such images by registered cataloguers.
US-RISM Libretto Database (13,000+ citations for libretti in the Schatz Collection at the Library of Congress), available through the University of Virginia Library's online catalogue, Virgo (include keywords such as "operas" or "librettos" to find records).
***Note that tThe Schatz Collection microfilm is held in the Yale Music Library, Microforms, Reel 1904.***
Free Reference Sources Online...
All Music Guide (lists of recordings, sound samples)