Secret Code Games for Clever Kids (R): More than 100 secret agent and spy puzzles to boost your brainpower

Secret Code Games for Clever Kids®: More than 100 puzzles to boost your brainpower (Buster Brain Games)

U.S.A. Secret Code Puzzles for Kids (Dover Kids Activity Books: U.S.A.)

Secret Code Breaking Puzzles for Kids: Create and Crack 25 Codes and Cryptograms for Children aged 6 to 10. Great as a Gift for Junior Spies