Teachers,
Maybe you would consider these books for your students:
Substitution Ciphers and How to Break Them
The Vigenère Cipher and How to Break It
Full disclosure: These are great books, and I wrote them myself, based on years of accumulated expertise in the the Cipher Challenge.
I can echo the recommendation. We were delighted when Madness offered us the text of his fantastic book on codebreaking to add to the BOSS library. Given that the pdf is available here for free, we are more than happy to recommend that those who can afford a printed copy of these sections for their (or their school’s) library should head over to Amazon to take a look! Harry