Nancy Springer 
Silver Sun 

As evil threatens a magical land, two brothers must find the strength and courage to stop it, in the second volume of this classic fantasy adventure series. In the Kingdom of Isle, where the Sun Kings reign with the power of the Book of the Suns, Hal and Alan are given a mission. They must use the ancient strength of wisdom to destroy the evil that plagues the kingdom. The two blood brothers venture throughout the land fighting the many forms that this evil takes so they can arrive at their destiny.