All those books that contain a scientific arrangement of the text, the formulas, and directions of the external divine worship or services of any church.
These books contain all the prayers and rubrics that a priest must say and observe when administering any of the Sacraments.
To this category belong also the liturgical regulations with regard to place (church, furnishings), time (ecclesiastical year, seasons), things (vestments, candles), etc.
The Liturgical Books are used as distinct texts in the theological curriculum.
A great number of the historical and illustrative references are taken from the numerous Sacramentaries, Lectionaries, Graduals, Psalters, Passionals, Homiliaries, Hymnals, and the special "Ordines" and "Consuetudines" of the various monasteries and dioceses.
Such books must always have the official approbation of the church for which they are written.
The sources for the composition of Liturgical Books are the Missal, Breviary, Martyrology, Pontifical, Roman Ritual, Ceremoniale Episcoporum, Memoriale Rituum, Octavarium Romanum, and the decrees of the Congregation of Rites (Decreta Authentica).
New Catholic Dictionary