Secret Agent Y.O.U. by Helaine Becker

This book is a lot of fun. You can take an exam to see if you’d make a good spy (Are you average looking so you could disappear in a crowd? Are you fit enough to escape pursuers?) You’ll read about the role of spies in history, learn how to disguise yourself, and to tell if someone is lying (hint: watch their body language). You can test your observation powers and learn the secrets of codes from easy to difficult.
The book tells you how to make fake blood (corn syrup and yogurt with food colouring) and how to give yourself a wart (glue mixed with bread!). Of course there’s a recipe for invisible ink. Here’s a tip for passing on a secret message: fold the message and put it under your dog’s collar. Walk the dog and meet your contact who asks if he can pet your dog. You say yes, and he removes the message from the dog’s collar, then both of you continue in opposite directions.