Sky Bound — James Morris

The Earth is split into three kingdoms:
One that resides in the sky, one that lies beneath the waves, and one that remains on the surface. Life on the surface is less than spectacular; in fact, it's quite the opposite. For fifteen year-old Sam Cutter, that's enough reason to risk everything and head for the Sky Nation. Sam has determined that nothing will stop him. The severe punishment for being caught leaving the surface has deterred all attempts at escape. If Sam can successfully flee the surface, he'll be the first to make it in ten years.
Heading into the unknown with little more than a bag filled with a few possessions slung on his back and pure determination, Sam turns and waves goodbye to his mom before sneaking onto a cargo ship bound for the Sky Nation.
Sam's goal of getting to the Sky Nation won't be easy. His journey takes him on an adventure filled with unexpected turns as Sam is abruptly thrown into the middle of a war, forced to choose a side, and stay alive. Sam learns to trust his instincts, adapt to new situations, and that he may be more important in this struggle than he could have ever known.