As we pointed out in a previous post, three of the Gospel Books under examination were formerly kept in book-shrines. The shrines are extant for the Book of Dimma (TCD; fig. 1) and the Book of Mulling (National Museum of Ireland), while the damage visible on Codex Usserianus Primus implies that it was also kept in a metal box for some time (see more on this HERE).
The practice of enclosing books in ornate boxes probably stems from the use of book caskets during religious ceremonies in early Christian Rome. The Gospel, considered to be the Word of God, needed to be housed in an appropriate manner: lavish bindings and boxes were devised to protect the Scriptures and assert their importance through the use of precious materials.