Books of Hours are like automobiles. While they consist of certain prayers and texts (those discussed in all the other chapters except this one) without which they cannot properly function nor be properly called Books of Hours, there was a nearly inexhaustible array of ancillary prayers that people, depending on their piety and their pocketbook, felt free to add. Medieval people personalized their prayer books the way modern people accessorize their cars (and for some of the same reasons).
One of the most frequently encountered accessory prayers is the “Joys of the Virgin” (fifteen is the usual number, although five, seven, and nine also appear); they celebrate the happy moments in Mary's life from the Annunciation to her Assumption into Heaven. Extra Hours (that is, in addition to those of the Virgin, Cross, and Holy Spirit) also appear: the Hours of St. Catherine, the Hours of John the Baptist, and the Weekday Hours (Sunday Hours of the Trinity; Monday, of the Dead; Tuesday, of the Holy Spirit; Wednesday, of All Saints; Thursday, of the Blessed Sacrament; Friday, of the Cross; and Saturday, of the Virgin). These ancillary Hours are short, structured like those of the Cross and of the Holy Spirit.
Since Books of Hours were used in church as well as at home, many contained Masses, the actual prayers and texts that were recited by the priest at the altar or sung by the choir. These Masses usually contain those texts that changed from feast to feast (Introit, Collect, Epistle, Gradual, Sequence, Gospel, Offertory, Communion, and Postcommunion), but they often include some of the unchanging parts of the service as well.
Another popular accessory text is the “Stabat Mater.” Its emotional intensity, rhythm, and rhymes make this prayer one of the most moving and memorable of the Middle Ages. Written probably in Franciscan circles in the thirteenth century, it spread quickly and by the late fifteenth century had been incorporated into the Church's official liturgy of both the Mass and the Divine Office. It has been set to music many times, most famously by Palestrina, Pergolesi, Verdi, Dvorák, and Poulenc.
Another accessory text, the ”Salve sancta facies” (Hail Holy Face), is a prayer to the holy face of Christ that was especially popular in late fifteenth- and sixteenth-century Flemish Horae. It was frequently accompanied by generous indulgences.