At first, the inclusion of the Book of Ruth (it doesn’t appear at all in the Lectionary for Mass) was curious, but she is the Gentile ancestor of King David.
147. During Advent, following an ancient tradition, passages are read from Isaiah in a semicontinuous sequence, alternating in a two-year cycle. In addition, the Book of Ruth and certain prophecies from Micah are read. Since there are special readings from 17 to 24 December (both dates included), readings for the Third Week of Advent which fall on these dates are omitted.
The following assignments cover from the day after Holy Innocents to the day before Epiphany. Remember this is the universal Roman Calendar, not the adaptation that places Epiphany on the Sunday after January 1:
148. From 29 December until 5 January the readings for Year I are taken from the Letter to the Colossians (which considers the incarnation of the Lord within the context of the whole history of salvation) and the readings for Year II are taken from the Song of Songs (which foreshadows the union of God and humanity in Christ): “God the Father prepared a wedding feast for God his Son when he united him with human nature in the womb of the Virgin, when he who is God before all ages willed that his Son should become man at the end of the ages. [Gregory the Great, Homilia 34 in Evangelia: PL 76: 1282.]
149. From 7 January until the Saturday after the Epiphany the readings are eschatological texts from Isaiah 60-66 and Baruch. Readings remaining unused are omitted for that year.
Not using the Office of Readings, I don’t know how the adapted Epiphany structure works for those who do pray it. This is one feast in which the moving around affects its liturgical surroundings several days in either direction.