It's well worth the money even at full price, but you can get it for significantly less via Amazon. It's aimed at relative beginners, and most particularly at younger ones, but assumes that readers are intelligent and don't need too many concessions, so is a satisfying and informative read for anybody. I enjoyed it, though naturally there were some details I'd quarrel with. The accompanying CD adds considerably to the package.

EFDSS has already been mentioned as an invaluable resource, but I'd point particularly to David Atkinson's extensive English Folk Song Bibliography, which includes helpful commentary on the entries:  http://efdss.org/songbib3.pdf. It covers general introductions to the subject as well as more specialised studies.