The Mysterious Benedict Society

I loved this book written by Trenton Lee Stewart. When a peculiar ad appears in the newspaper, dozens of gifted children enroll to take a series of mysterious and challenging tests. But in the end just four very special children will succeed. Their challenge: to go on a secret mission that only the most intelligent and resourceful children could complete. To accomplish this they will have to go undercover at the Learning Institute for the Very Enlightened, where the only rule is that there are no rules. As the children face physical and mental trials beyond their wildest imaginations, they have no choice but to turn to each other for support.

There are two sequels: Perilous Journey and Prisoner's Dilemma.