Seconding the recommendations so far. A Song for Every Season is lovely. The Folk Handbook is a relatively recent publication IIRC. Certainly good people were invloved in putting it together. The Penguin Book of English Folk Songs is now published as Classic English Folk Songs. Both are available from EFDSS. I am particularly fond of Traveller's Joy, an EFDSS publication focusing on the singing of Gypsies and Travellers. It comes with a useful CD with a selection of the book's songs.