I have to admit that I didn't read this very carefully-- the image-rich E ARC loaded rather slowly, and I'm not as interested in code as I was as a nine-year-old. (Did I make my own secret language? Check. Leave notes under rocks in that language for my brother? Check.) I was impressed that there was a nice overview of the history and use of codes, and then a wide variety of different ones. I imagine that all of these might end up in Jack and Max books. The Caesar code is in The Battle for the Emerald Buddha: Thailand, although I don't remember one in the second book. This book is a great motivator for children to stretch their brains over the summer and write notes in a variety of secret codes. This is a skill that is always good to have!