"Music must flow. It must be a thread from no one knows just where.
It must not be a problem in mathematics."
We recommend American Favorite Ballads, Vol. 5 and Folk Songs for Young People by Pete Seeger, as well as the book "To Everything There is a Season": Pete Seeger and the Power of Song. We also have (and love) brother Mike's True Vine. Below are the third and fourth movements of the quartet.
Further reading: Ruth Crawford Seeger: A Virtual Autobiography, by Judith Tick. Remembrances by Peggy and Mike appear here as well.