You never know what's going to set off a flurry of activity. Around here lately it's a thin, used paperback that Junior's dad brought home from a library sale: The Usborne Book of Secret Codes (1997). It cost all of 50 cents. Yesterday the three of us spent a good hour sending out important information, like "What would you like for dinner?," in pigpen code.
Roman numerals aren't code exactly, but are certainly intriguing to some would-be spies. (As far as I can tell, these aren't taught in elementary schools any more. Hence, the intrigue.) Agent J. has studied David A. Adler's Fun with Roman Numerals (Holiday House, 2008), copying over numbers and making notes to himself.
A printout of the Wikipedia entry on Morse code is well-used, too. I wouldn't be surprised if a homemade telegraph machine is in our future.