Six sublime singers from the early music choir Exsultemus, stars of the Boston scene, perform rare treasures from the Eton Choirbook. This music from around 1500 is virtually unknown today--it is what Henry VIII would have heard as a boy. The soaring lines and luminous sonorities are a breathtaking revelation of the English gothic style in its full flowering. The pure and glowing soprano voice of director Shannon Canavin floats above her five male companions for effects of devotional rapture. Earthier, secular music from other manuscripts enriches the program. A convivial reception follows.