The Ender's Game series, by Orson Scott Card. There are currently eleven books in the series, with a twelfth, Shadows in Flight, on the way.
Ender's Game and its sequel, Speaker for the Dead, both won Hugo and Nebula awards. My personal favorites are Ender's Game and its parallel novel, Ender's Shadow. These just might be the best books you will ever read, so savor them.