The following general works of music reference will serve in most instances as the best starting points for questions having to do with music terminology, theory, chronology, or bio-bibliography. Encompassing simple definitions as well as sweeping syntheses, they assist in the approach to large spans of material. For more specific subject reference sources consult the annotated guides listed below.
Baker's Biographical Dictionary of Musicians. 9th ed. Edited by Laura Kuhn [et al.]. New York:
Schirmer, 2001. 6 vols.
REF ML105 .B168B4 2001 Diccionario de la Música Española e Hispanoamericana. Director y coordinador general, Emilio Casares Rodicio; directores adjuntos, José López-Calo, Ismael Fernández de la Cuesta; secretaria técnica, María Luz González Peña. [Spain]: Sociedad General de Autores y Editores, 1999-2002. 10 vols.
The Encyclopedia of Popular Music. 4th ed. Edited by Colin Larkin.
The Garland Encyclopedia of World Music. Edited by James Porter
and Timothy Rice. New York: Garland, 1998-2001;Routledge, 2002.
10 vols. + 9 CDs.
The Grove Dictionary of American Music. 2nd ed.
Edited by Charles Hiroshi Garrett. Oxford: Oxford University Press, 2013. 8 vols.
The Harvard Dictionary of Music. 4th ed. Edited by Don Michael Randel. Cambridge, Mass.: Belknap Press of Harvard University Press, 2003.
Die Musik in Geschichte und Gegenwart: Allgemeine Enzyklopädie der Musik. 2te.
begründet von Friedrich Blume. Kassel;
New York: Bärenreiter; Stuttgart:
Metzler, 1994-2007. 29 vols.
The New Grove Dictionary of Music and Musicians. 2nd ed.
The Oxford Dictionary of Music. 6th ed. Edited by Michael Kennedy. Oxford: Oxford University Press, 2012.
REF ML100 .K46 2012
Reference resources on the Web are too vast--and unstable--to summarize in any meaningful way, and visions of fully monitored, deep-search aggregating metasites are still in the developmental stages. But for basic questions the following (nonsubscription) music reference tools may be of service:
African-American Music Reference 
Classical Music Reference Library
The Garland Encyclopedia of World Music Online
The Oxford Encyclopedia of Theatre and Performance
Oxford Music Online
RILM: Abstracts of Musical Literature
