Magyk ~ Angie Sage

This is the first book in the Septimus Heap series. Septimus is the seventh son of a seventh son, who dies shortly after birth, at the same time, his father finds a tiny baby in the forest, and brings her home to his wife and they raise her as their own. Ten years later many changes happen and the Heap family discovers that they have been raising the princess, and that Septimus may yet be alive!