Scratch Orchestra Book
Cornelius Cardew is the best! There is very little video footage of him with the Scratch Orchestra. But, recently a book was published by the author of the essay that's linked on this page, Stefan Szczelkun, which features studies on the concepts of 'Improvisational Rites.' It's titled "Improvisation Rites: From John Cage's 'Song Books' to the Scratch Orchestra's 'nature Study Notes'. Collective Practices 2011 - 2017."
You can find out more here:
http://stefan-szczelkun.blogspot.jp/2017/11/improvisation-rites-from-john-cages.html
Definitely worth a read!