@CeciliaJulia Another book you might enjoy is The Musical Shape of the Liturgy, by Dr. William Mahrt. He explains how the musical structure of the proper chants reinforce the dramatic character & structure of the Mass itself, as well as discussing polyphony, hymnody, and pretty much every possibility for liturgical music, in a collection of essays spanning over thirty years. It's wonderful reading, a deep well worth going back to repeatedly. (It can be downloaded for free at the above link.)