The Books of Magic #3 The Children's Crusade

Timothy Hunter is just like any other thirteen-year-old boy in London . . . except for the tiny fact that he might be the most powerful magician of his time.
There is a secret world for children in danger, and right now it needs Tim's power to survive. But how can he help when he is still trying to figure out how to use his magic - and when certain dark forces seem to have sinister plans of their own?