Use the search terms "vocal music" along with "choral music." This will ensure you get a larger results set.
Modernism and Opera edited by Richard Began and Matthew Wilson Smith [Ebook]
Singing with your whole self: a singer's guide to Feldenkrais awareness through movement (2nd edition) by Samuel Nelson
Vocal repertoire for the twenty-first century by Jane Manning (In two volumes -> Vol. 1 is pre-2000 music. Vol. 2 is post-2000 music)
