Books on classical song are at these call numbers:
Analysis and interpretation MT 115-121
Bibliographies Ref ML 128 .S3
Song Literature ML 2400-2862
Translations ML 54.6 (Reference and General Stacks)
See also composer's biographies: ML 410
Classical songs (also called "art songs") are primarily in collections at these call numbers:
Use the library catalog to find songs. The best way, particularly for recordings, is to search the song title as a phrase.
If you need the music for songs by major composers like Schubert, Schumann, Brahms, etc., it's often easiest to go to the shelf at M 1620 to see exactly which volume(s) you need.
Find sources for free online scores under the Scores tab above. Some other sites:
Books of song translations are at call number ML 54.6 in Reference and General Stacks. Recordings can also be a good source for translations.