Top ten: books with secret signs

Justin Scroggie is the author of Tic-tac Teddy Bears and Teardrop Tattoos. For the Guardian, he named a top ten list of books with secret signs.His introduction and one title from his list:I'm an author (and television producer) with a passion for secret signs – all the ways that people in the know privately communicate with each other. I love books where something hinges on a sign or a symbol