Otto von Weddigen's monumental two-volume "Geschichte der Theater Deutschlands" (1904) is available at the Internet Archive:

Vol. 1: http://archive.org/details/geschichtederthe01wedd

Vol. 2: http://archive.org/details/geschichtederth02wedd

Thanks to the University of Toronto for this most welcome scan.

Theatre Finder is supposed to become "a comprehensive, web-based, world-wide guide to theatres over 100 years old"

http://mith.umd.edu/theatrefinder/

See slate.com

"The Stunning Grandeur of the World’s Great Opera Houses"