Dress the characters in the highly specialised clothing required for their various secret missions. Including 12th century Turkish hashashins, Japanese ninjas, the British Parachute Regiment, Nepalese Ghurkas, Soviet Army Spetsnaz soldiers, US Navy SEALs and the SAS. With over 160 stickers.
My boys (5&4) ask to read this book all the time. While it's probably geared more towards 8 and up, as a read-a-loud it has been great. Descriptive, historical, and fabulous photographs.