Erak's Ransom (Ranger's Apprentice Book 7) - John Flanagan

Will, you're going to a place that is unlike anywhere you have ever seen. You have been sent on a mission to rescue the Skandian leader who has been captured by a dangerous desert tribe. Their land is beyond all your experience so far. Violent sandstorms, warring tribes and danger stalk you at every turn. Amongst your group is the princess, whose life you must protect. Can you keep her safe and reach the hostage before time runs out?