Top ten books with secret signs

In 2008 Justin Scroggie named a top ten list of books with secret signs for the Guardian. One title on his list:The Scarlet PimpernelIn Baroness Orczy's play and novels, The League of the Scarlet Pimpernel is a secret society of English aristocrats, dedicated to rescuing French nobility from the guillotines of the French Revolution. The League is led by the mysterious Scarlet Pimpernel (aka