Geraldine Mccaughrean 
Tamburlaine“s Elephants [EPUB ebook] 

RUSTI is a Mongol warrior, fighting for the bloodthirsty Tamburlaine, Conqueror of the World. He intends to show the enemy neither fear nor mercy… until he comes face-to-face with his first elephant.
KAVI is the elephant“s rider. Captured by the terrifying Mongol Horde, he fears for his life. But the boy who takes him prisoner does not kill him. And soon it seems they might almost become … friends.
Then Rusti uncovers a terrible secret, and the unlikeliest of friendships is put to the ultimate test.