When the altar is incensed during Mass, the Missal is temporarily removed. Anything else on the altar is incensed including altar cards, microphone, the priest’s spectacles and the deacon’s copy of the weekly Mass sheet, but never the Missal. We incense the Book of the Gospels or the Lectionary, but the only occasion on which the Missal is incensed is at the Solemn Entrance on Maundy Thursday before the Gospel is proclaimed from it.
Our ritual practices have a liturgical significance although many originally came about for purely practical reasons. The incense is blessed and its smoke conveys the blessing to the altar, the priest and the congregation: the rising smoke signifies our prayer rising to heaven. It’s original use was to keep everything smelling sweet and to keep the flies away during animal sacrifices in the temple. However I’m at a complete loss to understand why an altar server has to hold the missal while the altar is incensed. It cannot be because the smoke might damage the paper or we wouldn’t incense the Gospel.