https://www.atlasobscura.com/articles/what-is-a-girdle-book

Go Medieval by Attaching a Book to Your Belt

They’re incredibly rare today, but portable girdle books were once very handy.

A girdle book held by the Beinecke Library at Yale University. BEINECKE LIBRARY/CC BY-SA 2.0

GIRDLE BOOKS HAD TO BE small, and they had to be light. From the bottom edges of their bindings extended an length of leather, usually gathered into a knot at the end. This extension of the cover could be used to carry the book like a purse or could be tucked into a girdle or belt. To read, the owner wouldn’t even have to detach the book; when taken up, the book would be oriented correctly, just as if it had been pulled from a shelf.

Used from the 14th to 17th centuries, these books were texts that their owners needed to keep close at hand: prayer books used by monks and nuns, for example, or law books used by traveling judges. Though they were valuable objects—luxuries, even—these books were meant to be consulted and read.

“These are books that needed to be specially protected because of a lot of use, a lot of wear. Most of them were probably used daily,” says Margit J. Smith, author of The Medieval Girdle Book. “How many books do you have in your collection that you use every day?”