Set in a world of shape-shifting animals and armored polar bears, Pullman's trilogy, His Dark Materials, has become wildly popular over the past decade, selling more than 5 million copies in the US alone. In the wake of the success of the Lord of the Rings and Narnia franchises, it might seem like a no-brainer to use the books as the basis for another series of fantasy movies -- but this franchise comes with so many extra degrees of difficulty, it seems unbelievable that it ever got a green light.