Erak's Ransom by John Flanagan
Erak, the Skandian Oberjarl, has been captured by a desert tribe. Will, Halt, Horace, and Evanlyn set out to rescue him, but the desert is much different than anywhere they have been before. The maps they have do little good, since no one goes to the desert and there are many tribes with many strange laws. While kidnapping is normal for some of the tribes, Will and Halt soon realize that there is someone else behind this. Someone who has something against Erak and his alliance with King Duncan.