Post-apocalyptic fantasy in which the evil Overlords have taken over the Earth fifteen years before the book opens, at which time everyone over the age of fourteen disappeared. The remaining children are herded into the Dorms an used as material for making the Overlords' creatures and for breeding more children for the same purpose. On their fourteenth birthday each child is taken to the Meat Factory where they're transformed into things that are no longer human to fight in the Overlords' ritual battles.
Some few children escape the Dorms and an even smaller number find the sanctuary offered by Shade, a computer simulation of a human adult, who has made it his mission to fight the Overlords and restore the world to humanity. But even Shade isn't all he seems.
As might be expected from Garth Nix, a strong story with strong characters and plenty of twists and turns. The motives of the Overlords are rather sketchy, but since the book is from the point of view of four of the children who escape the Dorms, that's fair enough - the reader knows what the characters know and the Overlords don't tend to have heart to heart chats with humans. The ending was perhaps a tiny bit rushed after all the buildup, but that's the only real criticism I can muster.