This was an interesting 32 page non-fiction book about the history and structure of secret codes.  Various code styles are described along with techniques for cracking them.  It was fun to apply the techniques described in the examples along the way.  The answers were in the back to check progress.  The format was easy to read and included lots of pictures to keep things interesting.  I think students will enjoy reading this and writing their own secret codes.  It is one in a set of 3 in the series.  The other titles include Speaking Secret Codes and Making Secret Codes.