There are many music dictionaries and encyclopedias and, depending on the topic you are researching, you may need to consult a specialized one or a more general one. We've highlighted several of the core encyclopedias and dictionaries for vocal music below to get you started.
New Grove Dictionary of Music and Musicians by 
Harvard Dictionary of Music, 4th ed. by 
Encyclopedia of Musical Theater by 
A concise biographical dictionary of singers : from the beginning of recorded sound to the present by 
Großes Sängerlexikon by 
Dictionnaire des chanteurs de l'Opéra de Paris by 
