Someone on an MNRF forum just recommended this book for performers that is available through Google books.
http://books.google.com/books?id=vyPfW6FLB1IC&printsec=frontcover&dq=street+theatre+and+other+outdoor+performance#v=onepage&q=&f=false
Oooooh... that looks really interesting. Unfortunately it's not the whole book. You might be able to find it a university library,though.
The book is still in copyright, so you are both unlikely and illegal to find it online as a whole :)
It's a good book, but it primarily focuses on the European concept of "street theatre" which is much more "theatre" and much less "busker" as we tend to think of it. Back in the 60's, 70's and 80's, there were a lot of artistic installations, community festivals, etc. There still are quite a few!
The author, Bim Mason, teaches at Circomedia, a circus school in Bristol, UK.