Ender's Shadow by Orson Scott Card

From chapters.ca:
Andrew "Ender" Wiggin was not the only child in the Battle School; he was just the best of the best. Here is the story of another of those precocious generals, the one they called Bean -- the one who became Ender's right hand, his strategist, and his friend. Bean's past was a battle just to survive on the streets of Rotterdam. He was a tiny child with a mind leagues beyond anyone else's. Bean's desperate struggle, and his remarkable success, brought him to the attention of the Battle School's recruiters, those people scouring the planet for leaders, tacticians, and generals to save Earth from the threat of alien invasion. Bean was sent into orbit, to the Battle School. And there he met Ender...

A fantastic parallel story to Ender's Game that begins a series of its own. I do not know which book I thought was better, they were both amazing.