Break the Code: Cryptography for Beginners (Dover Children's Activity Books) / Bud Johnson

No, it's not a message from outer space! Just read it backwards, starting from the bottom line, to discover what this specially coded message says. (The extra letter "X" at the beginning was thrown in just to make the message a bit more mysterious.)
Codes have been used for centuries. Now, with the help of this entertaining book, you, too, will be able to decipher dozens of secret messages. Veteran cryptographer Bud Johnson not only describes the different techniques used in secret writing but also explains — clearly and simply — how to make and break six different kinds of codes: simple letter grouping, reverse trail, shift code, key word code, date shift code, and twisted path code. You'll even learn techniques for creating your own confidential messages!
Cartoons by illustrator Larry Daste accompany the 51 practice messages, along with easy-to-follow instructions explaining each type of code and how to use it. There's also a glossary defining common terms used by cryptographers, a sheet of reproducible graph paper for writing out messages, and a page of ready-made alphabets for shift and key word codes. And if all those funny drawings and helpful guidelines aren't enough to help you decode our secret messages, don't worry. Solutions are included at the end of the book.