The Garland Encyclopedia of World Music is the first comprehensive resource devoted to music research of all the world's peoples. Each of its 10 volumes is devoted to a different geographic area. Its full text is available online through Alexander Street Press' Music Online.
Baker's Biographical Dictionary of Musicians by This page highlights some of the many music dictionaries and encyclopedias that can serve as foundation for your research.
New Grove Dictionary of Music and Musicians
The New Grove Dictionary of Music and Musicians, 2nd edition is widely acknowledged as a pre-eminent authority on all aspects of music.
Some of the most useful features of the New Grove are the detailed bibliographies included with each article, which provide readers with additional resources for further research, and complete lists of works for all the major composers.
The full text of the New Grove is available through Oxford Music Online.
Other New Grove Dictionaries
New Grove also publishes several important, more specialized encyclopedic dictionaries. Their full texts are also available through Oxford Music Online.
Die Musik in Geschichte und Gegenwart
The Oxford Dictionary of Music by Also available through Oxford Music Online.
The Harvard Dictionary of Music by 
