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Volume 1 (Europe) has lengthy histories of the major movements in German and Austrian theater. Check out the Further Reading section at the end of each entry for scholarly books and articles about each country's theater.
Published more recently than WECT (2007). Check the extensive entries for Germany and Austria to get ideas for playwrights, plays, or movements. You can also check the bibliographies at the end of each entry to find scholarly books and articles about German and Austrian theater.
German Expressionist Theatre: The Actor and the Stage considers the powerfully stylized, anti-realistic styles of acting on the German Expressionist stage from 1916 to 1921.
The World Encyclopedia of Contemporary Theatre (WECT) by 
The Columbia Encyclopedia of Modern Drama by 
German Expressionist Theatre by 
Postdramatic Theatre by 