Laddertop, Books 1 & 2, Paperback

Out of deep space, a mysterious alien race known as "The Givers" came to Earth. They gave the human race the greatest technology ever seen-four giant towers known as Ladders that rose 36,000 miles into space. Each tower culminated in a space station that harnessed the energy of the Sun to power the entire planet. As suddenly as the Givers arrived, they vanished, leaving the human race with one solemn instruction: maintain and preserve the Ladders at all costs. Due to the unique alien construction of the Laddertop space stations, however, only a skilled crew of children could perform the maintenance necessary to keep the stations running. Twenty-five years later, back on Earth, competition is fierce to enter Laddertop Academy. Robbi and Azure, two eleven-year-old girls who are best friends, are among those vying for a spot at the prestigious academy. While one is rejected, the other takes off into space for the adventure of a lifetime. Yet soon, their destinies will collide, as they must decipher an alien message and solve an ancient mystery that could either save the Earth from invasion...or trigger its imminent destruction.