Follow this link to read a fascinating article by Paul Horn, a curator at the Wellcome Collection. He has been going through books in the Collection's archive for their digitisation project, and has found many intriguing items of ephemera folded into their leaves. As he says 'Books leave their traces in our minds, but often we leave traces of ourselves in books, too. These fascinating items found inside old works help us read between the lines of those who came before'.
https://wellcomecollection.org/articles/WpUguyUAAB8sBAyc