Question: Why do you move the missal back and forth on the altar (P.L., GA)
Answer: At Solemn High Mass the Gospel is sung by the deacon at a prominent place within the church, called an "ambo" or a "pulpit." The ambo is usually located on the north side of the sanctuary (the altar is assumed to be at the east end of the church). If no ambo is available, the subdeacon holds the book, standing in the north end of the sanctuary. The Epistle and other Lessons, being of lesser importance, are chanted on the opposite side of the sanctuary by a subdeacon or lector. The book is held by the reader, or placed on a podium of simpler appearance than the ambo. It is said that the transition from one side of the church to the other represents the transition of God's word from the Synagogue to the Church; a symbolism that fitted better in the early Church when Old Testament readings were more frequently read at Mass.
When the readings are recited or sung at the altar, as at Low Mass or a simple High Mass, the book is moved from one side to the other in imitation of the practice at Solemn Mass. The Epistle and Lessons are delivered at the right side of the altar, giving the name "Epistle side" to that side. The "Gospel side," is the left, and the missal is turned to the north-northeast, so that the priest can face in approximately the same direction as the deacon at Solemn Mass.
The book remains on the Gospel side throughout the Canon, perhaps because this is the place of honor, or, more practically, because the priest can more easily read the Canon with the missal on his left, while he makes the numerous signs of the Cross with his right hand (even if he is left handed).
The missal is returned to the Epistle side after the priest rinses his fingers and the chalice. This may simply be to return the book to its original location for the next Mass. It also distinguishes the Communion verse and Postcommunion prayer from the Last Gospel, but the late date at which this reading entered the Mass seems to disqualify this as a reason.