Reusing old vellum manuscripts in book bindings was standard practice back in the olden days (that's a technical term meaning before about 1700), and some rare texts are only known through fragments visible on bindings. These are very cool looking. You'll have book from the 1500s with lettering from the year 1000 running vertically across the endpapers or covers. No doubt there's a name for this sort of thing but I've forgotten it.