Solemn Mass (also known as a Solemn High Mass), is the full ceremonial form of the Tridentine Mass, celebrated by a priest with a deacon and a subdeacon, requiring most of the parts of the Mass to be sung, and the use of incense. The parts assigned to the deacon and subdeacon are often done by priests in vestments proper to those roles. (source)
During a Solemn Mass, the deacon carries the Book of the Gospels to the priest to be kissed. Then the deacon carries the Book of the Gospels outside of the altar rails (or another dignified and appropriate place) to read the Gospel.