Q. 1. What is the purpose of the "Book of Gospels?"

A. 1. The "Book of Gospels", also known as the Gospel Book or the Evangelion, is a codex or bound volume containing one or more of the four Gospels of the Christian New Testament, normally all four, centering on the life of Jesus of Nazareth and the roots of the Christian faith. The term is also used of the liturgical book, also called the Evangeliary, from which are read the portions of the Gospels used in the Mass and other services, arranged according to the order of the liturgical calendar.

Liturgical use in churches of a distinct Gospel Book remains normal, often compulsory, in Eastern Christianity, and very common in the Roman Catholic Church.

In its current usage, the Book of the Gospels or Evangeliary contains the full text of the passages from all four gospels that the deacon or priest is to read or chant at Mass in the course of the liturgical year. However, use of the Book of the Gospels is not mandatory, and the Gospel readings are also included in the standard Lectionary.

The Book of the Gospels, if used, is brought to the Altar in the entrance procession, while the Lectionary may not. When carried in procession, the Book of the Gospels is held slightly elevated, though not over the head. It is particularly proper for the deacon to carry the Book of the Gospels in procession, as the reading of the gospel is his particular province. When there is no deacon, the Book may be carried by a lector.

Upon reaching the Altar, the deacon or lector bows in veneration of the Altar, then places the Book upon the Altar, where it remains until the Alleluia.

During the singing of the Alleluia, the deacon (who before proclaiming the Gospel receives the presiding priest's blessing), or in his absence, a priest, removes the Book from the Altar and proceeds with it to the ambo. If incense is used, the Book of the Gospels is incensed by the deacon before the reading or chanting. An Altar server or acolyte will swing the censer slowly during the reading or chanting. The Book of the Gospels remains on the ambo until the Mass concludes, unless it is taken to a Bishop to be kissed, after which it may be placed on the credence table or another appropriate and dignified place.

If the Rite of Dismissal of catechumens is celebrated, the Book of the Gospels is carried in procession in front of the catechumens as they leave the church.

[Source: https://en.wikipedia.org/wiki/Gospel_Book]