The Cambridge Companion to Twentieth-century Opera begins with a chronological list of operas from 1900 to 2000. It's much longer than my list which really only covers about 30 years. In fact for every opera we know there are ten we've never heard of.
New works are commissioned, mounted, advertised, reviewed and forgotten year after year. It reminds me of my forgotten Italian composers list, but the Baroque Italians expected to provide new music each season and never to look back. Our expectations are a bit different. Each new opera is expected to be a masterpiece.
The companion book treats all these forgotten operas as though they had indeed been masterpieces, giving as much space to Malipiero as it gives to Poulenc. The last few chapters try to assess the future of opera and provide some interesting insights. If you're very, very studious, you might enjoy it.