Here's the official synopsis for Starglass:
Terra has never known anything but life aboard the Asherah, a city-within-a-spaceship that left Earth five hundred years ago in search of refuge. At sixteen, working a job that doesn't interest her, and living with a grieving father who only notices her when he's yelling, Terra is sure that there has to be more to life than what she's got.
But when she inadvertently witnesses the captain's guard murdering an innocent man, Terra is suddenly thrust into the dark world beneath her ship's idyllic surface. As she's drawn into a secret rebellion determined to restore power to the people, Terra discovers that her choices may determine life or death for those she cares most about. With mere months to go before landing on the long-promised planet, Terra has to make the decision of a lifetime—one that will determine the fate of her people.