Denis Arnold, et al, through Oxford Music Online. Includes concise articles on:
1. The origins of opera
2. From the Florentines to the 1640s
3. From the opening of the public opera houses to the 1730s
4. Serious opera in France until the Revolution
5. Rococo and reform in opera seria, 1725–90
6. Comic opera until the time of Mozart
9. French opera in the 19th century
10. Italy during the 19th and early 20th centuries
11. German opera from Mozart to Richard Strauss
12. Opera elsewhere in western Europe
13. Slavonic opera
14. The 20th century: Symbolism
16. Russian fantasy
17. Naturalism renewed
18. Myth and allegory
19. Music theatre versus opera
20. Opera today
2001, 2nd edition by Guy Marco. Presents 2,833 descriptions of books, parts of books, articles, and dissertations on the operas of 320 individual composers, and about operatic life in forty-three countries.
2008, 2nd edition edited by Stanley Sadie and Laura Macy. Includes a full synopsis of the plot, a cast list, a note on the singers in the original production, and information on the origins of the work and its literary and social background. Contributions conclude with a brief comment on the particular work's place in operatic history.
1987, by Gustav Kobbe. Presents excellent plot synopses and some descriptive analysis of a very large number of operas composed from 1607 through the early 1990s. Includes all of the standard operas and many lesser known works.