There is a very useful book published by the Elphinstone Institute The Bedesman and the Hodbearer comprising the correspondence between Walker and Child, and edited by Mary Ellen Brown.
Willam Walker (1840-1931) did a lot of the legwork and research in Scotland for Child while Child worked on the 'English and Scottish Popular Ballads'. He also lived long enough to assist Gavin Greig and James Duncan in their collecting work in the North-East of Scotland.
There is a good article by Ian Olson on Walker in the Dictionary of National Biography.

Matthew