For musicians who are looking for an authoritative reference source on the subject of music, The Grove Dictionary of Music and Musicians is probably the most authoritative one available in the English language.
According to an entry on Wikipedia on this dictionary, "The Grove Dictionary of Music and Musicians is an encyclopedic dictionary of music and musicians...and is the leading music reference source in English for both encyclopedic information and bibliographies". At the point in writing, the latest edition of this dictionary is named The New Grove Dictionary of Music and Musicians (2nd edition), edited by Stanley Sadie and John Tyrrell.
I find that The New Grove Dictionary of Music and Musicians is a very useful reference for one to start off with when one is doing research for the purpose of writing programme notes, or simply, to learn more about an established composer or a musical term.
There is an online edition of The New Grove Dictionary of Music and Musicians. Here's the URL for Grove Music Online: http://www.grovemusic.com
(Note: Subscription to its services would be necessary to access the database.)
For readers who are living in Singapore, The New Grove Dictionary of Music and Musicians (2nd edition) is available for reference purposes at the following public libraries: library@esplanade, Lee Kong Chian Reference Library, and Woodlands Regional Library.