Try,
Fairportfolio by Kingsley Abbott for the late 60's. There is another book but that takes things up to the early 80's
Theres a biography of Sandy Denny by Clinton Hylan (??spelling)around, again lots of Fairport input but other snippets regarding the late 60's.
I believe that Ashley Hutching has also got a biography due out, again fairport period but may be useful to you.
Cheers
Sledge