Christian liturgical books usually contained specific Gospel or Psalm readings for specific days in the liturgical year. These books made it a bit easier for an officiating priest or bishop to read what was needed for any particular mass. Psalters contained readings and passages from Psalms, while Evangelaries contained readings from the Gospels and, occasionally, from other books of the New Testament. These books would have been used during the mass itself, in front of the congregation, making them some of the more visible books of the Middle Ages. In the Greek Church, the menology might be used to relay both scriptural readings appropriate to the day and lives of saints that might be read or noted during a worship service.
Büttner, F.O. The Illuminated Psalter: Studies in the Content, Purpose, and Placement of its Images (Turnhout: Brepols, 2004)
Olson, Mary. Fair and Varied Forms: Visual Textuality in Medieval Illuminated Manuscripts (New York: Routledge, 2003)
Sandler, Lucy Freeman. “The Illustration of the Psalms in Fourteenth-Century English Manuscripts: Three Psalters of the Bohun Family,” Reading Texts and Images: Essays on Medieval and Renaissance Art and Patronage in Honour of Margaret M. Manion. ed. Bernard J. Muir (Exeter: University of Exeter Press, 2002) pp. 123-151
Toswell, M.J. The Anglo-Saxon Psalter (Turnhout: Brepols, 2014)