This is interesting: a flickr set of photographs of drawings and writings found in books in jails in Wisconsin.
I've never found such artwork in my library's books, but I have been keeping all the scraps of paper and other miscellaneous debris I find around the library. I've wanted to display them all somehow, because they are interesting, but I just haven't figured out how to do it. A flickr set might be the best way - look for that in an upcoming (but not immediate future) post.
This was found via BoingBoing.