Phil Edwards wrote -
"Is there a ballad-by-ballad commentary on Child anywhere? I'm thinking of something that would go through each ballad printed by Child & document.... where Child got it from (as far as we know)

There is none that I know of but might not Bertrand Harris Bronson's 4-volume The Traditional Tunes of the Child Ballads or even his abridged 1 volume The Singing Tradition of Child's Popular Ballads be the sort of thing that you are seeking.

Both are available from Pete Shepheard's website by clicking here. Of course, you may have to re-mortage your house to get them for as Pete points out - "second hand copies of the four volumes have been fetching well over £1000."