A [[strap]] fastened onto a single [[book]] to prevent it from being opened and damaging the inner pages, or used on a [[book stack]] to secure them for easier transportation. Commonly made from [[leather]] and has a [[buckle]]. Usually seen on [[grimoire]]. 

For book strap that is attached to a [[belt]] or [[shoulder strap]] for the purpose of carrying the book around hands-free, use [[book holster]].
book strap