Grove Music Online is comprised of the New Grove Dictionary of Music and Musicians, the New Grove Dictionary of Opera, and the New Grove Dictionary of Jazz. It includes extensive and scholarly articles with excellent bibliographies. Print versions of these dictionaries are also available in the reference section of Underhill Library.
The Oxford History of Western Music by Richard Taruskin (ML160.T18 2005 v.1-6) is a good resource for placing a piece in context both historically and musically.
Xreferplus Online allows for cross searching over a number of music dictionaries including the Harvard Dictionary of Music and the Harvard Biographical Dictionary of Music. The articles are not as detailed as Grove but worth exploring.
Sound recording program notes that come with compact discs and LPs can sometimes contains some very good information about the piece you’re working on. Search Tripod, the Tri-College Online Catalog and try using the “music” search limited to “sound recordings”. Also try the Naxos Music Library online, a database of thousands of streaming audio files, many of which come with an "about this recording" link to program notes.
Editor’s notes in the preface to the edition of the music you are working from might have some good notes. If your edition doesn’t then perhaps another edition in the library does. Again, search Tripod and try using the “music” search limited to “music scores”.
Tripod, the Tri-College Online Catalog is a good place to search for books about composers and their works. Try a subject search on the composers’ last name. You can also try a keyword search combining the last name of the composer and the type of music you’re playing; for example, bach and sonatas and flute. If you limit the results (by using the “limit” button) to material type books and location Swarthmore, you’ll have a shorter list to browse.
Shelf browsing is also useful. Composers biographies can be found in the call number area ML 410 and are arranged alphabetically by the composer’s last name.
Music notation: a manual of modern practice by Gardner Read (currently on course reserve for Music 13 MT 35.R253 M9 1972)
Sounds and signs: aspects of musical notation by Hugo Cole (ML431.C64)
Naxos Music Library Glossary, a text resource within the Naxos Music Library of online sound recordings.
A Dictionary of musical terms in four languages by W. J. Smith (Reference ML 109 .S66)
Dictionary of musical terms by Vincent Picerno (Reference ML 108 .P57)
A Dictionary of vocal terminology by Cornelius Reid (Reference ML 102 .V6 R4 1983)
Lied & Art Songs Texts Page has tens of thousands of song texts, many with English translations. Entries can help answer questions such as who wrote the text, what was the original language of the text, who else has set this text.
The ring of words; an anthology of song texts by Philip Miller (ML54.6.M5 R5 1973)
Lyrics and translations included with notes to sound recordings are also great resources for texts and translations.
Analysis of nineteenth- and twentienth-century music, compiled by Arthur Wenk (Z 6811 .M98 no.25, 1987).
Music criticism; an annotated guide to the literature, by Harold J. Diamond. (Reference ML 113 .D5).