D.J. MacHale 
Pendragon: The Rivers of Zadaa [EPUB ebook] 

The struggle for good versus evil continues as Bobby Pendragon follows the demon, Saint Dane, to the territory of Zadaa, home of the Traveller, Loor. Zadaa is on the brink of all out war and Saint Dane is ready to push things over the edge — unless Bobby can stop him. But, the rules have changed, and with the destruction of the flume on Eelong leaving two fellow Travellers trapped, can our 21stcentury hero find the strength to outwit an evil tyrant and bring Zadaa to a truce?