These are reference resources. They are great to refer to at the beginning of a project, especially if you're new to the subject. Encyclopedias will give you a concise and readable introduction to a particular musician, style, genre, etc.
However, reference sources typically do not present a distinct point of view, and they are less useful when you are constructing an argument.
Grove Music Online (Part of Oxford Music Online)
The primary English-language reference source for music. Covers music of all styles, periods, and geographic regions.
Garland Encyclopedia of World Music Online
More than 700 articles covering the music of the world's peoples, from the earliest known traditions to current practices.
Harvard Dictionary of Music, 4th ed. Don Michael Randel, ed. Cambridge, MA: Belknap Press of Harvard University Press, 2003
An authoritative source for definitions and concise articles on musical topics and terms. No articles on people. Online access is through Credo Reference Library.
Location of library's printed copy: MUSICMEDIA REF ML100 .H37 2003
Continuum Encyclopedia of Popular Music of the World. Ed. by John Shepherd et al. New York: Continuum, 2003– . 7 vols.
Encyclopedia of Popular Music, 4th ed. Don Michael Randel, ed.
Now available online through Oxford Music Online. Search this on its own or with other Oxford titles.
Covers popular music, including world music, from 1900 the present, with articles on artists, genres, trends, record labels, venues, and festivals.
Oxford Encyclopedia of the Music of India. Published in 2011.
Covers music of India, Nepal, Bangladesh, Pakistan, and Sri Lanka. Includes articles on technical terms, dance styles, instruments, and styles of music from classical to film-originated.
Print version: MUSICMEDIA REFERENCE ML101.I47 O94 2011 (3 vols.)
Oxford Bibliographies Online. Music.
Guide to scholarly literature in music. The initial release contains articles on 50 topics and individuals, with extremely useful bibliographies of recommended resources. Cross-searching with other modules of Oxford Bibliographies Online is possible in Advanced Search. Highly recommended if the available articles pertain to your topic.