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.
Truncation symbol: *. Wildcards: ? for one character; * for one or more characters (c*t finds cat, cut, carat, clout, count . . .)
Take advantage of the bibliography at the end of most articles.
To see all articles listed alphabetically, click on "All Content" at upper left of home page.
To find the primary article on a topic, use the search boxes at top right, and change All to Title. (Gagaku in Title finds the article titled "Gagaku.")
For Advanced Search, click the Search button in the top gray bar.
Use the Browse function for All Volumes to see a list of audio examples online (Click box beside the volume’s title.), a list of articles (Click box beside Body.), and the Guide to Publications in back of volume (Click box next to Back.)
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
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.