Ender's Shadow - Orson Scott Card
Having read and enjoyed Card's Ender Wiggins quartet, I was advised (by Chris, also a fan of the series) to try Ender's Shadow. Instead of being a prequel or sequel, this one is a parallel novel to the original Ender's Game. That is, it takes place at the same time as the original but centers on a different character—this time, Bean, the tiniest soldier in Ender's army.
If you haven't read any of the Ender series, it's one of my favorite science fiction works. The first is actually quite popular for young readers, too, but then the following books are less space-adventure and more philosophical.
I suppose part of the reason I enjoyed both books so much is that I identify with Ender and Bean, kids who got picked on for being too small and too smart. I also really loved the concept of the parallel novel: the only other example I can think of is the play "Rosencrantz & Guildenstern Are Dead," Tom Stoppard's spoof of "Hamlet," but this one isn't so lighthearted. The books do not entirely overlap, since Bean's story takes him in a few different directions, but the scenes that do take place in both books are fascinating. Instead of Ender's inner thoughts, this time we have only Bean's, and the conversations take on a different tone.
The book is written to stand on its own, even if you haven't read any of the other novels, but if you do that, I would recommend reading it again after you've read the first four to have a fuller understanding. (But also, one of the surprises from Ender's Game is given away in this book, so perhaps it's best to read them in order.)