Liar & Spy by Rebecca Stead

A quiet, observant boy who has recently moved into an aging apartment building joins a small neighborhood “spy” club with a charismatic new friend to surveil a mysterious upstairs tenant; as their game of stakeouts and codes deepens, the boys confront bullies, unravel misunderstandings, and learn difficult truths about trust, compassion, and what it means to tell the truth — discovering along the way that people are more complicated and lonely than they first appear.