The English Plainchant Revival by Bennett Zon
I'm honestly not experienced enough with plainchant to really understand some of the charts they give or the distinctions between certain ways of resetting the shapenote manuscripts to modern notation.
However, I can appreciate the history.
It's actually a bit amazing the plainchant kept any sort of place in England considering the penal laws against Catholicism during the 17th and 18th centuries.
I liked the quote used just before the first chapter:
'Q. What is the use of singing and of organs in the divine service?
A. To help to raise the heart to heaven, and to celebrate with greater solemnity the divine praises.' (Richard Challoner, 1758-81)