This archive, from a quick glimpse, looks like a wonderful resource.
I searched ballet and limited the search to text and the resulting full text items included:
Bournonville And Ballet Technique (1961)
Author: Erik Bruhn; Lillian Moore
Publisher: The Macmillan Company
The Russian ballet (1913)
Author: Terry, Ellen, Dame, 1847-1928
Subject: Ballet; Dance -- Soviet Union
Publisher: New York, Indianapolis : The Bobbs-Merrill company
Book contributor: University of California Libraries
The Internet Archive
"The Internet Archive is a 501©(3) non-profit that was founded to build an Internet library, with the purpose of offering permanent access for researchers, historians, and scholars to historical collections that exist in digital format. Founded in 1996 and located in the Presidio of San Francisco, the Archive has been receiving data donations from Alexa Internet and others. In late 1999, the organization started to grow to include more well-rounded collections. Now the Internet Archive includes texts, audio, moving images, and software as well as archived web pages in our collections."