The un-magician - Christopher Golden

Timothy has spent his entire life outcast from the world on the island of Patience. Here Timothy is kept safe from a world that his father, one of the most powerful men in a world of magic, believes will not accept him. Timothy is a weakling, a freak of nature and the most dangerous boy in the world. He was born completely powerless in a world governed by magic. When his father dies, Timothy is brought into the world of magic and taken in by the Society of Alhazred, the same society to which his father belonged. Here his defect is useful. Timothy is undetectable by any magical means.. the ultimate spy. When he engages in espionage on the Society's behalf, he feels accepted and loved, but he doesn't realise he is risking his life - the Society is using him as a pawn in a plot to overthrow the government and reinstate a mad sorceror at its head!