Stumbling across something the kids have been doing on their own is one of my very favorite things about family life. I found this on the couch.
I was fascinated by this book as a child and delighted to find it for a dollar at the library book sale.
These are great books about secret codes:
Lu & Clancy's Secret Codes by Mason & Cupples
Mysterious Messages: A History of Codes and Ciphers by Blackwood
The Usborne Book of Secret Codes by O'Brien & Riddell
Let's Investigate Codes and Sequences by Smoothey & Baum