Starminster by Megan Hopkins

Astrid has lived in the rhubarb shed her whole life. The outside world is dangerous, Mama explains; she will understand when she’s older, but she cannot set foot beyond the door. Astrid longs to see the world; to meet other children; to live in the farmhouse with Mama.

But what she longs for most of all is to see the stars. Then one night, a stranger unlocks the shed: a stranger with wings. She has come to take Astrid to a secret city in the sky called London Overhead, far above the highest peak of the Shard.

For Astrid, like her, is a Librae – and will soon grow wings of her own . . .