Below are key reference books that provide a general overview of a topic or help identify synonyms, related terms, or basic data. These sources often include references and lists of further readings.
Willamette University's Book of Hours (Link to digital copy)
This Flemish Book of Hours, entitled Praeces piae, made in third quarter of the 15th century in the diocese of Tournai, in modern Belgium. The text provides prayers for the eight hours of the day at which prayers were recited (Matins, Lauds, Prime, Terce, Sext, None, Vespers, and Compline).