Ender's Shadow
Ender's Shadow is a 1999 book (ISBN 0812575717 for paperback edition, ISBN 031286860X for hardcover edition) by Orson Scott Card with a plot covering the events in Ender's Game but from the point of view of Bean, a child genius who is largely peripheral to the action in Ender's Game.
Bean, the main character in the book, is an orphan living in the hellish streets of a future Rotterdam, after escaping at the assumed age of one from a Dr. Volescu's (His first name is never used) illegal genetic testing facility. His brilliant mind, with capabilities far beyond those of anyone else he encounters, brings him to the attention of Sister Carlotta, a nun working to recruit children for Battle School, though as an ulterior motive is striving to get children off of the streets and give them an education. This story takes you through Bean's experiences in Battle School and shows how Bean, though seemingly peripheral in Ender's Game, was much more important to the war, Battle School, and largely to the fate of Earth, than he seemed. The book reveals important insights and discoveries including Petra's explanation of her "betrayal" and how Dragon Army was chosen.
Ender's Shadow is the first of a series that includes Shadow of the Hegemon, Shadow Puppets, and Shadow of the Giant.