John Christopher 
Dragon Dance 

Simon and Brad face their greatest danger yetand discover the secret of the fireballin the third book of the alternative history Fireball Trilogy, from critically acclaimed Tripods author John Christopher.When Simon and Brad were caught in the mysterious fireballa whirling ball of light that sucked them into a bizarre alternate worldthey knew a terrifyingly different future was ahead. The world on the other side was geographically identical to their own, but the societies and history were completely new…and full of danger. Discovering a pagoda in California was strange enough, but when Simon and Brad are captured by slavers headed to China, the future seems even more hazardous. Especially when they are banished to a remote mountain retreat with the mysteriously powerful Bei-Kun…and come face-to-face with the secret of the fireball.