There are a number of short liturgical utterances scattered throughout Book of Hours, typically indicated by rubricated abbreviations. Below I list the most common ones, with a brief discussion of its function.
A or Ant.: Antiphon. a short liturgical text that often introduces or closes a psalm or prayer. The text itself may be abbreviated to its opening words.
Ben.: Benedictus. A blessing prayer, biblical in origin.
Cant.: Canticle. A sung biblical text. The most common ones are the Magnificant, the Nunc dimittis, and the Benedictus dominus deus
Hymn.: Hymnnus. A song, usually verse. Much more variable than the other liturgical utterances
Inv.: Invitatory. An antiphon used as a refrain, esp. in Matins.
Lec. Lesson. A spoken (not sung) reading, often biblical
O., Ora, or Oratio: Oration. A prayer
Ps.: Psalm. One of the texts from the Book of Psalms in the Old Testament. Term is sometimes also applied to canticles.
R. Response. A phrase spoken or chanted in reply to the versicle. In a Mass, the response would be spoken by the congregation or choir.
V. Versicle. A phrase spoken or chanted (in a Mass, by the priest) to which there will be a Response.