For anybody thinking of buying the book, let me describe some of the research Harker undertook. He did indeed interview a number of people. Foremost among these were Peggy Seeger, whom he interviewed a number of times.   

Others include a number of other critics group members, including Michael Rosen and Sandra Kerr. For me, the fact that he spoke to different people and presented differing views is one strong feature of the book; you get a variety of points of view.

Here is a list (probably incomplete) of other people also interviewed by the author, over a period of several years. He plainly put a lot of work into the book.

Karl Dalas, Calum MacColl, Neill MacColl, Hamish MacColl, Clive Barker, Jean Newlove/MacColl, Vic Gammon, Wizz Jones, Rosalie Williams.

He also had access to the Seeger/MacColl archives and to tapes of the Critics Group.

His list of references runs to pages.