Books which have interested me include "The Ballad as Song" by Bertrand Harris Bronson, which IMO encourages/provokes thinking about the songs in their variations; also "The Ballad of Tradition" by G. H. Gerould, which covers the basics in clear English; and for the most concise introduction to ballads, I would refer a newcomer to Emily Lyle's introductory essay in her smashing collection called "Scottish Ballads", published by Canongate Classics.
And, in addition to Wimberly, there is "The Silver Bough" by F. Marian McNeill, which deals with Scottish lore.
Of course, it is quite possible to sing a ballad without reading any of these books - but I find that some knowledge opens out the ballad for me and 'tunes' me in to the mindset of previous generations of singers.