Speaker For the Dead - By Orson Scott Card
This is the second book in the Ender universe, although I wouldn't really call it a direct sequel. It is based long after the first and has an almost entirely different feel to it.
However, I read this before I first read Ender's Game and I feel it manages to maintain the quality perfectly well. I can't assure that if you liked Ender's Game you'll like this as much, since they are certainly different in style. Even though it is separate, there are many things which will make you feel at home if you read Ender's Game.
Overall Rating: 9/10