The Waterson harmonies are strictly oral based on their early family experiences of hymn etc singing, so as far as I know, and I've been good friends with all of their families since 1966. I strongly recommend getting hold of the Topic 'Mighty River of Song' set of CDs with a booklet that outlines their history and influences. A book would certainly be an excellent undertaking. last I heard, American, Greer Gilman, was working on something but whether it came to fruition I don't know. I haven't seen her for 20 years.