Maths libraries should have The Code Book by Simon Singh and In Code by Sarah Flannery. The first is a history of codes and code-breaking, and explains the maths involved. In Code is about the author, who developed an encryption method. It includes lots of mathematical puzzles.
Why not organise a visit to Bletchley Park, where the Enigma code was cracked (www.bletchleypark.org.uk)?
Simon Singh's website (www.simonsingh.netThe_Black_Chamberhome.html) has activities and simulations on codes and code-breaking.