I thought this was fun:
Secrets revealed by dirty books from medieval times from the BBC
The scientists whose work is profiled in the piece mean books with actual dirt on the pages, which says something about the reading habits of well, at least those people in the middle ages who could actually read and had access to books, which was probably not so many of them.
Add this type of research to the list of things that would be lost if e-readers take over. (Though with printing presses and mass-produced books, even these days it might be that only one person will ever read a particular copy of a book.)