Encyclopedias, Dictionaries, Guides, etc.
Try these first...
Oxford Music Online (Includes: The Oxford Dictionary of Music, The Grove Music Online, The Oxford Companion to Music) [more info]
Harvard Dictionary of Music (via Credo Reference-provides definitions of musical terms) [more info]
Also try...
Baker's Biographical Dictionary of Music (via Biography Resource Center) [more info]
Contemporary Musicians (via Biography Resource Center) [more info]
Dissertations & Theses - Full Text [more info]
see also Interdisciplinary databases-Dissertations
Emusicquest: Music In Print (scores) [more info]
The Faber Companion to 20th Century Popular Music (via Credo Reference) [more info]
The Garland Encyclopedia of World Music Online [more info]
Harvard Biographical Dictionary of Music (via Credo Reference) [more info]
Index to Printed Music [more info]
International Dictionary of Opera (via Biography Resource Center) [more info]
The Penguin Dictionary of Music, Sixth Edition (via Credo Reference) [more info]
RISM A/II: Music Manuscripts after 1600 (more info)
RISM A/II: Music Manuscripts (1600-1800) in British and Irish Libraries
Free Reference Sources Online...
All Music Guide (lists of recordings, sound samples) [more info]
Cantus: A Database for Latin Ecclesiastical Chant [more info]
Doctoral Dissertations in Musicology-Online [more info]
Music Theory Online Dissertation Index [more info]
Encyclopedia of Music in Canada [more info]