Erak's Ransom Ranger's Apprentice Series, Book 7.

What does it mean to earn the Silver Oakleaf? So few men have done so. For Will, a mere boy, that symbol of honor has long felt out of reach. Now, in the wake of Araluen's uneasy truce with the raiding Skandians comes word that the Skandian leader has been captured by a dangerous desert tribe. The Rangers are sent to free him. But the desert is like nothing these warriors have seen before. Strangers in a strange land, they are brutalized by sands...