The Lost Books series - book 3
One of the Chosen Has Gone Renegade.
Turning his back on all that he once believed, Billos does the forbidden and enters into a Book of History. He lands in a reality as foreign to him as water is to oil - a place called Paradise, Colorado. He has strange new power given to him courtesy of a mysterious figure known as Marsuvees Black.
The chosen four have survived the desert, escaped the Black Forest, battled the Horde, and added a spirited refugee to their number. But nothing has prepared them for the showdown that Billos, the renegade, is luring them into.