Seacliff, I regret to say it's highly unlikely there'll be any more opera books (although I'll probably slip in another short story or two). The reason is the research; I loved doing it, but it was so time-consuming that it began to interfere with that little thing called "Earning a Living".
At one time I was thinking of three more, with a new set of singers, with the last book set in 1935. That's the year that Gatti-Casazza retired as General Manager of the Met -- the one continuing character in all the books. That seemed like a good stopping place. But as much as I'd like to do it, I just don't see it in the works. Dang.