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.