Along with the websites there are lots of great books available secondhand (or from libraries that haven't got rid of their copies yet.) I wouldn't be without:
"The Ballad Tree" by Evelyn Kendrick Wells,
"Folklore in the English and Scottish Ballads." by Lowry C. Wimberly.
"Anglo-American Folksong Scolarship Since 1898." by D.K. Wilgus.

Several others given in the bibliographies of these books are also excellent. Good hunting, Joy