Mediaeval Liturgical Commentaries

Most people, when the subject of mediaeval liturgy is mentioned, think of the old service books which have the words spoken (sacramentaries, lectionaries, missals, breviaries) and descriptions of the action (ordos). There is another class of book entirely, the liturgical commentaries, which goes through the public services, explaining the elements they contain. They were also interested in the ministers of the liturgy, their orders and dress, in the structure of church and altar, in the calendar (temporal and sanctoral), and of course in the vexed question of how a priest computed the date of Easter (no diocesan journals in those days).