Ender in Exile by Orson Scott Card

Ender in Exile - Although written after several other of the Ender books, most of Ender in Exile happens between the 14th and 15th chapters of Ender's Game.  Ender chooses to join a colony ship to help settle one of the new planets won in the war against the Formics.  Val, his sister, accompanies him while she writes the history of the bugger wars.