Any form of communication is a medium for coded messages. Steganography through e-mail, code words exchanged during visitors' hours. I think it's silly to hold up books as a dangerous avenue for secret communication when there are already so many other ways of communicating with someone in prison.

The only way to prevent secret messages from getting inside prisons would be to cut off all communication with the outside world, and that would be totally inhumane even if it were possible.