The "book of hours" is a devotional book that was used by Catholics during medieval times to guide their daily prayers. This book is also known as a "horae", "primer", "missal", or "breviary". The term "horae" comes from the Latin word "hora," meaning hour, which referred to the designated time of prayer. "Primer" is a term used to describe a simplified version of the book of hours that was used by children and beginners. "Missal" and "breviary" were also devotional books used by Catholics during different periods of history. Today, the "book of hours" is considered a valuable piece of art and history, with surviving copies being highly sought after by collectors and scholars alike.