The Book of Durrow is a 7th century manuscript of the Christian Gospels. It was created at Durrow Abbey in Ireland (or possibly Northumbria, Northern England). The book is made up of 248 vellum sheets as well as six carpet pages, which are pages of art including geometrical shapes and animals, circular knots, triskeles, and spirals.
Durrow Abbey was a famous school, one of the "Universities of the West" of those times. The abbeys in that part of Ireland were frequent targets of Viking raids but somehow the Book of Durrow survived, until it disappeared after the abbey disolved in the 16th century.
Where the book went after that, no one knew but a century later it was found in the possession of a farmer (who stored it in his cow shed).
What and where was the Book during that century? That is a mystery that could consume a party of time-traveling adventurers, a quest for a group of medieval monks, a secret text for a cotery of hermetic mages or the solution to a epic mission for a bunch of young untried knights.