My BookPad cipher seems to be closely related to the running key cipher, since both take a long piece of text from a book and use it as key to encrypt a plaintext. Yet while the running key cipher can be broken easily, BookPad offers a level of security comparable to that of a one-time-pad. In this article, I try to explain why in layman’s terms.
A little spoiler: not much prevented the guy on the left from discovering this, back in the early XVI century. Had he discovered it, history might have turned out quite different.