The English and Scottish Popular Ballads, Francis J. Child — pretty much the definitive scholars’ source of ballads. Neatly indexed, with comparisons to related ballads and folk tales, for your singing pleasure.
Our Singing Country, Lomax & Lomax — American songs and ballads of all types, from all populations. Includes a lot of stuff that Child wouldn’t have looked twice at. The notation can be a little difficult to follow sometimes, because it attempts to faithfully reproduce all the variations in tune and meter that you get in field recordings. But it’s still more of a songbook and less of a stuffy scholar’s tome than Child.