The Wikipedia entry on cryptography has a fairly approachable entry going over the main terminology used in this chapter (and a lot more).
The encryption methods used these days rely on fairly advanced maths; for this reason books about encryption tend to either be beyond high school level, or else are about codes that aren't actually used in practice.
There are lots of intriguing stories around encryption, including its use in wartime and for spying e.g.
How I Discovered World War II's Greatest Spy and Other Stories of Intelligence and Code (David Kahn)
Decrypted Secrets: Methods and Maxims of Cryptology (Friedrich L. Bauer)
Secret History: The Story of Cryptology (Craig Bauer)
The Codebreakers: The Comprehensive History of Secret Communication from Ancient Times to the Internet (David Kahn) – this book is an older version of his new book, and may be hard to get.
The following activities explore cryptographic protocols using an Unplugged approach; these methods aren't strong enough to use in practice, but provide some insight into what is possible:
War in the fifth domain looks at how encryption and security are key to our defence against a new kind of war.
The book "Hacking Secret Ciphers with Python: A beginner's guide to cryptography and computer programming with Python" (by Al Sweigart) goes over some simple ciphers including ones mentioned in this chapter, and how they can be programmed (and attacked) using Python programs.