The Book of Hours is a religious text and could be considered a functional prayer book used for daily prayer often followed by the canonical hours which are appointed times of daily Christian prayer. The often illuminated or decorated images on the page were intended to encourage meditative thought and devotion. Often the Book of Hours guided prayers were used not only for prayer but to help with literacy at the time.
Stein A Wendy.“The Book of Hours: A Medieval Bestseller." The Met, https://www.metmuseum.org/toah/hd/hour/hd_hour.htm
Editors of Encyclopedia Britannica. “Book of Hours.” Encyclopedia Britannica, https://www.britannica.com/topic/book-of-hours-prayer-book