The New Grove Dictionary of Music and Musicians, available both in print and through Oxford Music Online, is the best and most comprehensive encyclopedia of music in the English language. It covers music terms and musicians, and includes works lists for major composers. A bibliography can be found at the end of most entries.
The publishers of The New Grove Dictionary have also published several subject-specific works. These include:
The Grove Dictionary of American Music
The Grove Dictionary of Musical Instruments
The New Grove Dictionary of Jazz
The New Grove Dictionary of Opera
The Norton Grove Dictionary of Women Composers