The Mysterious Benedict Society By Trenton Lee Stewart

This book is a spy novel, a book of enigmas, and a touching friendship novel all at once. Four unique orphans (or runaways) are brought together by a newspaper advertisement to complete a series of tests designed to test their minds. All decidedly brilliant children, they must ensue on a drastically dangerous mission to save the world- but first they must learn to get along. Solving mind-bending puzzles and narrowly escaping predicaments, these extraordinary kids are tested in every way in their struggles to find where they belong.
I WHOLLY recommend this book to anyone. Anyone! 
(This book is also a series in which the books get better and better as they go along. READ IT.)
This is the cover of the first book. You've probably already seen it around.