Q: Is there any commentary or instruction available that could explain how different canticles and psalms were chosen for the various offices? For example how did Psalm 63 come to be placed in “Sunday, Week One,” used so often? Or the Canticle of the Three Young Men
A: You’ll find less than you want to know in the General Instruction of the Liturgy of the Hours beginning at paragraph 126. You’ll find more than you want to know in Robert Taft’s book, The Liturgy of the Hours in East and West.