Laddertop / Card, Orson Scott

Eleven-year-old best friends Robbi and Azure are excited when they are accepted into Laddertop Academy to be trained to work on the Ladders, technology given to the human race by aliens that supply power to the entire planet and the space stations they support. Although only one of them makes it to the Ladders, the girls eventually end up working together to solve an ancient mystery that could mean either the end or the salvation of the human race.

Eleven-year-old best friends Robbi and Azure are excited when they are accepted into Laddertop Academy to be trained to work on the Ladders, technology given to the human race by aliens that supply power to the entire planet and the space stations they support. Although only one of them makes it to the Ladders, the girls eventually end up working together to solve an ancient mystery that could mean either the end or the salvation of the human race.

When the alien Givers came to Earth, they built giant towers that rose 36,000 miles into space leading to space stations that powered the Earth. After the aliens disappeared, only highly skilled children could perform maintenance to keep the power flowing to Earth. Two of those children, Robbi and Azure, become entangled in the mystery that may solve the mystery of the Givers.