Michael Haas's Forbidden Music: The Jewish Composers Banned by the Nazis is being published on 7 May and is an absolute must-read for anyone with an interest in setting the record straight about the nature of music in the 20th century. It is, to the best of my knowledge, the first major scholarly book to address its specific question. I have a short feature about it in The Independent today.
Here to go with it is a spot of appropriate opera: Korngold, in longing-for-the-past mode. This is "Mein Sehnen, mein Wähnen, es träumt sich zurück" from Die tote Stadt - the Pierrot Tanzlied, sung in this (unfortunately uncredited) production by [shock] a Pierrot.
I'll never forget hearing Olaf Bär in Zurich having to sing this dressed in drag with basque, tights, six-inch black heels and butterfly wings, but that's another story.