Here are a couple references that may overlap what you already have. I don't know which Sharp you have but "One Hundred English Folksongs For Medium Voice" by Cecil Sharp (Dover 1975, originally 1916) has a number of Child ballads
"The Ballad Book of John Jacob Niles" by J J Niles (Houghton Mifflin Company 1961) is made up entirely of deriviatives of Child ballads.
"The Singing Tradition of Child's Popular Ballads" by Bertrand H Bronson (Princeton University Press 1976). I have not seen this, only references to it.