That's the book I was thinking of. Definitely advice more appropriate for the concert hall than the liturgy, but I find many of my experiences in each area help me in the other!
Singing in Latin is (I think) still out of print, and worth a trip to the library. Copeman subsequently wrote or co-wrote the Latin chapters in Singing Early Music(ed. McGee) which go into less detail, but the book treats vernaculars as well, in case you want to add the mediaeval Catalan Sibyls to your chant repertoire.
To participate in the discussions on Catholic church music, sign in or register as a forum member, The forum is a project of the Church Music Association of America.