Jim,
The part of the Mass called LIturgy of the Word goes back to the begining of the Church, when bible readings were chosen to instruct the catechumens, who then left before the Eucharistic part of the Mass.
Until the liturgical reform, readings were distributed rather haphazardly throughout the year. When the so-called Novus Ordo mass was introduced, in 1969, it was decided to go through the whole bible (but not every passage) over a period of three years. These years are called a,b and c. Thus over a period of three years, we listen to representative samples of all the books of the Bible. If I remember right, we are now in year a.
Verbum