There was a whole series of math films and math books that seemed to enter popular culture at the same time. I'm sure this won't be the only book on codes. I'm not sure what triggered this. Just one of those things that's in the air. It seems to me that an interest in codes is actually pretty widespread.
Children love making up codes and creating secret little languages.
Lots of people love the idea of being part of some select group with its own secret passwords, like the Freemasons. So it's quite natural that children do this. And it's empowering. One of the reasons children make codes is to hide things from their parents. To children, parents are almost like Big Brother.