What's the matter with today's experimental music? Organised sound too rarely heard by Leigh Landy (1991), reviewed in NMA #10, has a discussion on definitions of experimental music and avant garde. Most of the text is on Google Books.
Landy advocates primarily a definition where innovation is the main characteristic of the music, above (but not necessarily excluding) all other factors. He also says 'innovation' needs to be distinguished from 'renewal'.