The Burning Bridge (Ranger's Apprentice Book 2)

Bracing for a final clash with the evil warlord Morgarath, Will and his friend Horace are chosen by the Rangers as special envoys to nearby Celtica, but Morgarath and his legions have found a way over the once impassible eastern mountains and are planning to ambush the king's army, and with the village of Celtica abandoned, only Will and Horace stand in the way of the dark lord's plans.