By about the middle of the 4th century there were certainly some liturgical books, How long before that anything was written one cannot say. The first part of the liturgy to have been written appears to have been the Diptychs. The word Diptych is derived from the Greek for twice­folded. A Diptych consisted of two tablets [covered with wax at the beginning] hinged and folded together like a book. On one the names of the living for whom prayers were to be said were written, on the other the names of the dead. These names were then read out by the deacon at the appointed place in the liturgy. Their use, in the East went on till far into the middle ages. Then the lessons were set down in a book. The old custom of reading from the Bible until the bishop made a sign to stop, soon gave way to a more orderly plan of reading a certain fixed amount at each liturgy. Marginal notes were added to the Bible showing this. Then an Index giving the first and last words of the amount to be read is drawn up. Other books were read besides the Bible [lives of Saints and homilies in the Divine Office]; a complete Index giving references for the readings is the “Companion to the books,” comes, liber comitis or comicus. Lastly, to save trouble, the whole texts are written out as they are wanted, so we come to the [liturgical] Gospel­book (evangelarium), Epistle­book (epistolarium), and finally the complete Lectionary (lectionarium). St. Jerome [324-420] is widely believed to have been commissioned by the pope to select the Epistles and Gospels used for each Sunday of the liturgical year, which have been used since in the traditional Roman Missal. ⁵ Meanwhile the prayers said by the celebrant and deacon are written out too.

A short History of the Roman Mass by Michael Davis.

www.Romancatholicism.org  The early Catholic Liturgy.