Try Heart of Albion - as an imprint they have an impressive stock of fine books. Especially fine is Marcia MacDermott's Explore Green Men (which explodes the myth of the Green Man as a folkloric image) and Bob Trubshaws Explore Folklore (though he loses the plot rather in his discussion of folk music, which deals with Genre Folk rather than exploring musical usage in vernaular social context as his main thesis would have suggested).

Otherwise:

Bob Pegg - Rites & Riots (and Folk, if you can find it).

David Thomson - People of the Sea

George Ewart Evans - everything

Ronald Hutton - Stations of the Sun

Steve Roud - The English Year

Bob Copper - A Song for Every Season

The Faber Book of Popular Verse

The list is endless...