I found this article and liked it because it gave me a lot of information about the Beale Cipher and treasure, even more information than Singh. I especially was interested in the reasons describing the possibility of it being real or a hoax.
Here's a list of 10 unsolved cipher texts throughout history, #2 is the Beale Papers covered in the chapter, #8 is Kryptos (the background of our website). Some date back as far as 1400 BC and some are more recent like the Zodiac Killer.
Here's the paper that Robert Ward of Maryland shared with me via email last year. The stats look solid to me, although I didn't check his p-values. Let me know if you work this into any of your projects this fall!