Sorcerer of the north. John Flanagan. 2009.

Will is finally a full-fledged Ranger with his own fief to look after. The fief seems sleepy - boring even - until Lord Syron, master of a castle far in the north, is struck down by a mysterious illness. Joined by his friend Alyss, Will is suddenly thrown headfirst into an extraordinary adventure, investigating fears of sorcery and trying to determine who is loyal to Lord Syron. As Will battles growing hysteria, traitors and most of all, time Alyss is taken hostage, and Will is forced to make a desperate choice between his mission and his friend.