Liar and Spy by Rebecca Stead

Liar and Spy is a book about a boy named Georges that moves into a new apartment and meets Safer, a boy his age that runs a spy club. Safer is determined to teach Georges everything about being a spy. Their latest mission is to spy on someone called Mr.X, he lives in the apartment above Georges. But as they find out more about their Mr.X plan, the line between games, lies, and reality begin to blur.