Mysterious Benedict Society by Trenton Lee Stewart

 "Are you a gifted child looking for special opportunities?" When four brilliant (and very different) orphans respond to a curious ad in the newspaper they find themselves caught up in an adventure none of them could have imagined. Recruited by the eccentric Mr Benedict, they help to uncover (and stop) a nefarious plot to take over the world.! Full of riddles, espionage and a whole lot of adventure this is one of my all time favourite books. - Amelia