From The Guardian:
Bindings made between the 15th and 18th centuries often contain hidden manuscript fragments that can be much older. Bookbinders used to cut up and recycle handwritten books from the middle ages, which had become old-fashioned following the invention of printing. These fragments, described by Kwakkel as “stowaways from a distant past”, are within as many as one in five early modern age printed books.
Access to such “hidden libraries” has been made possible by macro x-ray fluorescence spectrometry (MA-XRF), which allows pages to be read without removing the bookbinding.
(Dr Erik) Kwakkel added: “Much of what we’re finding is 15th or 14th century, but it would be really nice to have Carolingian material, so from the ninth century or even older. It would be great to find a fragment of a very old copy of a Bible, the most important text in the middle ages. Every library has thousands of these bindings, especially the larger collections. If you go to the British Library or the Bodleian [in Oxford], they will have thousands of these bindings. So you can see how that adds up to a huge potential.”