In the beginning, there was Ender's Game. This was a novel (actually, a short story, and then a novel) about a child genius named Ender Wiggin who became the best general the international Battle School had ever seen, and ended up saving humanity from conquest by an alien species.
It then spawned a trilogy of sequels--Speaker for the Dead, Xenocide and Children of the Mind--although that's not technically correct because the author, Orson Scott Card, actually intended Ender's Game as a prequel to that story, not vice versa. And after that, Card went on to other things.
But Ender's Game remained popular, and so, in time, Card revisited it. But not with more sequels. He wanted to tell the story of what happened to unite the world under Ender's brother, Peter Wiggin the Hegemon, between Ender's Game and Speaker for the Dead. This story was initiated during Ender's Game and briefly touched on in its epilogue, and that was it, because Speaker for the Dead begins long after Peter Wiggin is dead.
So Card came up with the fairly clever idea of taking a minor character from Ender's Game, another child genius named Bean, and retelling the entire events of Ender's Game from his perspective. You know how the plot is going to turn out, of course, but you don't know much of anything about Bean's origins, his friends or enemies, or what happens to him after humanity is saved and the Battle School is closed. And it turns out the little snot has a far more interesting history than anyone could have imagined....
That's what Ender's Shadow is about: Bean. And much like Ender's Game, this book isn't quite supposed to stand on its own. Because all the characters from Bean's life continue on into a new trilogy of books, beginning with Shadow of the Hegemon, about Bean and Peter Wiggin and everyone they know. No Ender, no Valentine (Ender's and Peter's sister), but a whole bunch of leftover adolescent military geniuses from Battle School trying to hang on in a world that wants nothing better than to kidnap them all and use them to win World War III.
See, it was pretty well set up in Ender's Game that the nations of the world were on the brink of some pretty nasty nationalistic infighting (yes, again), and that the looming threat of an alien invasion was the only thing that got them to put their differences aside and combine their resources. Once the aliens were dealt with, though, all those countries would go back to doing what they did before. Peter Wiggin foresaw this, even before his brother Ender went to Battle School. The fact that he was successful in preventing it by becoming the new ruler of the world himself himself was kind of glossed over before. Exactly how he did it, and how Bean plays into it all, is what these books are going to be about.
As a book, it's pretty good. And you don't need to read Ender's Game before reading Ender's Shadow, according to the author, but it certainly helps.
Ender's Shadow is followed by Shadow of the Hegemon, Shadow Puppets and Shadow of the Giant.