When first beginning a research project, it's a good idea to get background material on your topic. Reference works can save you a lot of time. They have done the work of compiling information about a person or topic for you - this can give you some context for the topic you are studying and help you frame your research question. They also have useful bibliographies that can jump start your research process.
Dictionaries and encyclopedias can be found for broad areas of music and for very specific subjects, so its best to start big and work your way smaller as you narrow your focus.
The New Grove Dictionary of Music and Musicians by 
Die Musik in Geschichte und Gegenwart. allgemeine Enzyklopädie der Musik (MGG) by 
The Harvard Dictionary of Music by 
New Grove Dictionary of Women Composers by 
International Dictionary of Black Composers by 
Baker's Biographical Dictionary of 20th Century Classical Music by 
A Chronology of American Musical Theater by 
The Encyclopedia of Popular Music by 
Encyclopedia of Recorded Sound by 
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