When the liturgy was extensively revised in 1970, one of the themes was the inclusion of a far wider range of biblical readings. At Mass, this meant a three-year cycle of Sunday readings and a two-year cycle of weekday readings. In the Office of Readings in the Liturgy of the Hours this meant a two-year cycle, both of Scripture readings and of the patristic Second Readings which accompany them.
The two-year cycle covers the whole of salvation history and uses practically every book of the Bible – not avoiding tricky passages which need thorough reading and meditation and aren’t suitable for the “listen fast or it’s gone” nature of the readings at Mass. It is also carefully designed to be out of step with the Mass readings, so that if you hear a passage read at Mass then it won’t appear in the Office of Readings for a year (or at worst, for a few months).
This masterpiece is lovingly described in §§147 to 152 of the General Instruction on the Liturgy of the Hours. But if you look in the actual printed books – it isn’t there. In its place is a one-year cycle of readings, covering half the material.