Fascinating, easy-to-understand book for would-be cryptanalysts of all ages teaches how to invent and decipher a host of codes and secret messages, including the "Moving Ahead" cipher used by Julius Caesar in his secret dispatches; dots and pinpricks, as used by the secret Rosicrucian society; and even a system used by Sherlock Holmes involving open spaces between important words. Includes stimulating brain-teasing projects that include all the codes described in the book. 14 b/w illus.