One of the ways that you prevent old books from decaying is to clean their parchment pages with a dry eraser. The waste removed in this process contains micro-fragments of the parchment material (which contains DNA from the cattle whose skins went into make it), DNA from people who have touched it, and the DNA of microorganisms that have collected on its surface.
This information, combined with other information about the object, can provide an additional window into the past.
Researchers recently did this analysis with the York Bible, which was written several decades before the Norman Conquest of England in 1066 CE, was probably written in 990 CE, and continues to be used by the Anglican Church (now, mostly as a devotional object rather than as a practical text).
The most interesting conclusion that they reached is that the York Bible's parchment came mostly from female calves, even though usually male calves are used to make parchment because male calfs aren't important for producing milk or expanding the herd. This was probably due to the fact that there was a major die off of cattle in England a couple of years before the book was written, due to a cattle disease, which left an excess of calf hide from both genders which could be used to make parchment.