Significant electronic and print dictionaries and encyclopedias

New Grove Dictionary of Music and Musicians
Music Reference Shelf under call number ML100 .N48 2001

The New Grove, second edition, includes entries on persons and subjects.  While predominantly a classical music resource, it now includes jazz, popular, and world music.  Also available electronically as Grove Music Online, which is part of Oxford Music Online. The landing page will say Oxford Music Online, but you will be searching Grove:
Grove Music Online
The electronic version of the New Grove Dictionary of Music and Musicians incorporates the New Grove Dictionary of Opera and the New Grove Dictionary of Jazz.

_____________________________________________________________________________
Die Musik in Geschichte und Gegenwart (MGG)

comprehensive music encyclopedia in German

Music Reference Shelf under call number ML100 .M92 1994  

This second edition is divided into two sections, the Sachteil (terms) and Personenteil (people).  The 1994 in the call number was the date the first volume was issued, but the rest of the encyclopedia was issued piecemeal and was finally completed in 2007.
MGG Online was launched in 2016. 
MGG Online
Please note: MGG Online now has Google translate imbedded which will translate articles into many different languages.  
______________________________________________________________________________

Harvard Dictionary of Music

authoritative dictionary of musical terms; available electronically through Credo Reference
Print editions:

Music Reference Shelf under call number ML 100 .H37 2003

Please note: the original edition, which came out in 1944 (ML 100 .A64, also on Music Reference Shelf) was called the Harvard Dictionary of Music. The author was renowned musicologist Willi Apel. It was updated several times (for example, ML100 .A64 1969, also on Music Reference Shelf). The new edition of 1986 (ML100 .N485 1986, also on Music Reference Shelf) was renamed the New Harvard Dictionary of Music.  The latest edition which came out in 2003, however, is once again called the Harvard Dictionary of Music.
________________________________________________________________________________
The following are all available as online databases from Credo Reference:

The Faber Companion to 20th Century Popular Music

The Harvard Biographical Dictionary of Music

Metropolitan Opera Encyclopedia: A Comprehensive Guide to the World of Opera

The New Penguin Dictionary of Music