Today on EWTN, they were showing Ash Wednesday Mass and the choir chanted the ordinary Latin Gregorian Mass parts, but they were done with maybe two, three, or four part harmonies. (Btw, I don’t know what is the name of this collection of Mass parts, but it’s the one that can be found in OCP… is it just the Mass proper?).
I know the scores for these parts are easily available online, but I can’t find any that have the additional polyphonic arrangement. Know where I can find it?
Are you talking about organum? The simplest form of organum is taking the chant line and transposing it to a fourth or a fifth and chanting it in parallel to the original line. (So you just have two lines.) I’m not sure if there is a book which shows this by musical notation. Whenever we’ve done it, we just have one person or one group chant the line a 4th/5th up or down from the main line and then the other person or group chant the main line. But you can do multiple lines in parallel organum, which is pretty cool. It’s pretty easy to do. Just give each group the note that they have to start on.
There are more complex forms of organum and it is incredible to hear. The first time I heard it a group of priests concelebrating the mass chanted in organum the consecration and other parts of the mass. It was hauntingly beautiful. But again, I’m not sure if there is a book showing the notations.
Edit - Just a note, organum is just an early form of polyphony when people began experimenting with harmonization and such in both sacred and secular music of the time.