Cryptography: An Introduction (2011) [pdf]
cs.umd.edu
cs.umd.edu
Covers most everything in the context of war history as far back and shaving a person's head, writing a message on it and then letting the hair grow back to hide it. Great read that will lay the groundwork of interest for a lot of people.
I can confirm that the content is very approachable. It even provides a great section introducing some of the core mathematics required.