ThomasForrest Kelly 
Chant and its Origins [PDF ebook] 

The Latin liturgical music of the medieval church is the earliest body of Western music to survive in a more or less complete form. It is a body of thousands of individual pieces, of striking beauty and aesthetic appeal, which has the special quality of embodying, of giving voice to, the words of the liturgy itself. Plainchant is the music that underpins essentially all other music of the middle ages (and far beyond), and is the music that is most abundantly preserved. It is a subject that has engaged a great deal of research and debate in the last fifty years and the nature of the complex issues that have recently arisen in research on chant are explored here in an overview of current issues and problems.