The Last Battle by C.S. Lewis

NARNIA... where dwarfs are loyal and tough and strong—or are they? ...where you must say goodbye... and where the adventure begins again. The unicorn says that humans are brought to Narnia when Narnia is stirred and upset. And Narnia is in trouble now: A false Aslan roams the land. Narnia's only hope is that Eustace and Jill, old friends to Narnia, will be able to find the true Aslan and restore peace to the land. Their task is a difficult one because, as the Centaur says, "The stars never lie, but Men and Beasts do." Who is the real Aslan and who is the imposter?