The Sorcerer of the North by John Flanagan
Will is finally a real ranger-his apprenticeship is done-so he has his own fief to look after. Nothing is happening in his fief and Will is quite bored. But suddenly, he gets reassigned to a castle in the north, where the lord has been struck with a mysterious illness. Local residents say that the illness is the result of sorcery. In disguise, Will travels to uncover the mystery.
When he first arrives, the solution seems simple: the lord's son is teaming up with the evil sorcerer, Malkallam, who lives in the woods. But after further investigation, Will realizes that superstition has a lot to do with the hasty conclusions everyone has been making. And no one in the castle is as they seem.
**Book 5 in Ranger's Apprentice**
Previous book: The Battle for Skandia
Next book: The Siege of Macindaw