Full-text is available through JSTOR and much of IIMP. Look for the icon to check for full-text availability in the other indexes.
ProQuest provides full-text online access for many dissertations and theses and RILM provides abstracts for dissertations and theses.
A great place to begin any music search is to check The New Grove Dictionary of Music and Musicians, 2nd ed.
A print copy of the music encyclopedia can be found in REFERENCE at ML100.N48 2000.
Grove Music Online is part of the Oxford Reference collection. In addition to The New Grove Dictionary of Music and Musicians, 2nd ed. (which also contains the texts of The New Grove Dictionary of Opera and The New Grove Dictionary of Jazz), you can also search The Encyclopedia of Popular Music, The Oxford Dictionary of Music, and The Oxford Companion to Music.
If you want to search all the sources at once, just type your term(s) in the "Search by Source" search box, or you can choose to search only Groves or one of the other sources by clicking in the box to the left of the title.
Richard Taruskin's Oxford History of Western Music is also a good source to check.
In reference: REF ML160.T18 2005
In the stacks: ML160.T18 2010 (yep, you can check this one out!)
Baker's Biographical Dictionary of Musicians REF ML105.B16 2001
Dictionnaire de la musique REF ML100 .D65
Enciclopedia della musica REF ML100 .E45
Encyclopédie de la musique et dictionnaire du Conservatoire REF ML100 .E5
Diccionario de la música española e hispanoamericana REF ML101.S7 D53 1999
The Harvard Dictionary of Music REF ML100.A64 2003
Die Musik in Geschichte und Gegenwart REF ML100 .M92 1994
Curated by a community of music scholars, students, teachers and librarians, the Open Music Library brings together peer-reviewed journal articles, books and music scores from the world’s digital collections. Many of the resources are open access (free to view) and others are by subscription only.