"A Tor/Seven Seas Paperback"--T.p. verso.

Twenty-five years ago, the alien Givers came to Earth. They gave the human race the greatest technology ever seen -- four giant towers known as Ladders that rise 36,000 miles into space and culminate in space stations that power the entire planet. Then, for reasons unknown, the Givers disappeared. Due to the unique alien construction of the Laddertop space stations, only a skilled crew of children can perform the maintenance necessary to keep the stations up and running. Back on Earth, competition is fierce to enter Laddertop Academy. It is an honor few students will achieve. Robbi and Azure, two eleven-year-old girls who are the best of friends, are candidates for the Academy. They will become entangled in a dangerous mystery that may help them solve the riddle of the Givers ... if it doesn't destroy the Earth first!