Libelii are small books more like pamphlets with only a few pages containing some liturgical texts. These are collection of formularies for one or several masses or various occasions. Although Libelii are not liturgical books as such they serve as an intermediary link in the transition from the period of improvisation to the formulation of the liturgical book, properly so called. Nonetheless, the first liturgical books are nothing but a collection of several libelii, which were formerly independent from one another. Libelii were organized with homogeneous content which served as antecedents for later development of sacramentaries and missals. Several studies on libelii shed light on the different forms of Christian worship in the early centuries.
 E. Palazzo, “Le role des libelii dans la pratique litugique du haut Moyen Age: Histoire et typologie,” Revue Mabillon 62/1 (1990): 9-36.