Magonia

Maria Dahvana Headley's soaring YA debut is a fiercely intelligent, multilayered fantasy where Neil Gaiman's Stardust meets John Green's The Fault in Our Stars in a story about a girl caught between two worlds . . . two races . . . and two destinies.
Aza Ray Boyle is drowning in thin air. Since she was a baby, Aza has suffered from a mysterious lung disease that makes it ever harder for her to breathe, to speak—to live. So when Aza catches a glimpse of a ship in the sky, her family chalks it up to a cruel side effect of her medication. But Aza doesn't think this is a hallucination. She can hear someone on the ship calling her name.
Only her best friend, Jason, listens. Jason, who's always been there. Jason, for whom she might have more-than-friendly feelings. But before Aza can consider that thrilling idea, something goes terribly wrong. Aza is lost to our world—and found, by another. Magonia.
Above the clouds, in a land of trading ships, Aza is not the weak and dying thing she was. In Magonia, she can breathe for the first time. Better, she has immense power—but as she navigates her new life, she discovers that war between Magonia and Earth is coming. In Aza's hands lies fate of the whole of humanity—including the boy who loves her. Where do her loyalties lie?