The Ancient Plain-Song of the Church: Adapted to the American Book of Common Prayer is a book written by Thomas Helmore and published in 1855. The book is a guide to the ancient plain-song of the church, which is a type of music that was used in religious services during the medieval period. The book provides an adaptation of this music to the American Book of Common Prayer, which is a book of liturgy used by the Episcopal Church in the United States...