Eda Kuhn Loeb Music Library - Harvard College Library

Online Resources for Music Scholars - Maintained by the Eda Kuhn Loeb Music Library, Harvard College Library

Digital Scores and Libretti - Online scores from the Harvard Library collections

Finding Songs in Collections - Maintained by the Eda Kuhn Loeb Music Library, Harvard College Library

Poetry & Music - Courtesy of the Academy of American Poets

Dictionaries and Encyclopedias

Grove Music Online - Part of Oxford Music Online; also provides access to The Encyclopedia of Popular Music,The Oxford Companion to Music, and The Oxford Dictionary of Music.
Die Musik in Geschichte und Gegenwart: allgemeine Enzyklopädie der Musik
Virginia Tech Multimedia Music Dictionary

Bibliographies and Indexes of Settings

Bradley, Carol June. Index to Poetry in Music: A Guide to the Poetry Set as Solo Songs by 125 Major Song Composers. New York: Routledge, 2003. 
Carman, Judith E. et al. Art Song in the United States, 1759-1999: An Annotated Bibliography. 3rd ed. Lanham, Md.: Scarecrow, 2001.
Clifton, Keith E. Recent American Art Song: A Guide. Lanham, Md.: Scarecrow Press, 2008.
An index of music composed or published after 1980
Gooch, Bryan N. S. and David S. Thatcher. Musical Settings of Early and Mid-Victorian Literature: A Catalogue. New York: Garland, 1979.
Hovland, Michael, comp. Musical Settings of American Poetry: A Bibliography. New York: Greenwood, 1986.
An index of published settings of poems by 99 American authors
Seaton, Douglass. The Art Song: A Research and Information Guide. New York: Garland, 1987.

The Lied and Art Songs Text Page

The Lyric Book: Complete Lyrics for Over 1000 Songs from Tin Pan Alley to Today. Milwaukee: Hal Leonard, 2001.

Phillips, Lois. Lieder Line by Line, and Word for Word. New York: Oxford University Press, 1996. Texts and translations of poems set by Beethoven, Brahms, Mahler, Schubert, Schumann, Strauss, Wagner, and Wolf

Stokes, Richard, ed. and trans. The Book of Lieder: the Original Text of Over 1000 Songs. London: Faber, 2005.

Lyrica Society for Word-Music Relations