Liar & Spy by Rebecca Stead
Georges (the s is silent) has a lot going on. He's having trouble with some boys at school, his dad lost his job and so his mum has started working all the time - and they had to sell their house and move into an apartment.
But moving into the apartment block does bring one good thing - Safer, an unusual boy who lives on the top floor. He runs a spy club, and is determined to teach Georges everything he knows. Their current case is to spy on the mysterious Mr X in the apartment above Georges. But as Georges and Safer go deeper into their Mr X plan, the lines between games, lies, and reality begin to blur.