The other side of the sky / Amie Kaufman, Meagan Spooner.

Janus is a world divided: above the surface float the gleaming sky-cities held aloft by massive, technologically advanced engines. In the shadows beneath is an older, darker world, full of flooded byways and ancient temples governed by magic and faith. When Prince North of Ciel falls from the sky in a terrible glider malfunction, one person sees his fiery descent like a falling star: Nimh, a goddess incarnate, searching for a way to save her struggling people. His fall fulfils an ancient prophecy in the world Below, and the two strike up a wary alliance - Nimh, so that she can fulfil her destiny and save her people, and North so that he can return to the other side of the sky and take his place as heir to the throne. As North struggles to believe in the superstition and spellcraft rampant on the world's surface, Nimh discovers a terrifying truth about the prophecy binding them together: that North's fall may foretell the end of both worlds. With renegade magicians and fanatic cultists out for Nimh's blood, and the ever-looming question of whether North's fall was accident or betrayal, the two must stay alive long enough to seek the truth about both their worlds. Forbidden by divine law to touch, but bound together by fate and the increasingly-strong pull of their own hearts, they forge a path together toward the fate of the world: to save it, or to end it...

Adolescent.