Annals of Opera, 1587-1940
by
Alfred Loewenberg, comp.
Baker's Dictionary of Opera
by
Laura Kuhn
The Metropolitan Opera Encyclopedia
by
David Hamilton, ed.
Opernlexikon = Opera catalogue = Lexique des operas = Dizionario operistico
by
Franz Stieger
These extensive educator guides include a variety of resources such as synopses, in-depth classroom activities, musical snapshots, suggestions for discussion, composer timelines, streaming audio excerpts, and more
Opera & Ballet Primary Sources (OBPS) is hosted at Brigham Young University. You can search this database for digital scores, Libretti, and other related documents.