The Sorcerer of the North
Years after a great victory, Ranger Will is settling into his new fief, but peace is short-lived. When Skandian invaders appear, Will must defend his land. His duties soon lead him on a secret mission fraught with danger, where he must uncover traitors and face down fears of sorcery. With the king weakening and his friend Alyss captured, Will faces a difficult choice between his mission and his loyalty.
The Sorcerer of the North is the fifth book in the Ranger's Apprentice series. While each book has a self-contained adventure, the overarching character development and world-building benefit greatly from reading the series in order to fully appreciate the narrative.