By Garth Nix.
One day fifteen years ago, seven Overlords appeared and everyone over the age of 14 vanished. The children were herded into Dorms until their fourteenth birthdays, then to be taken as collections of useful parts and made into biological monsters for the Overlords' war games.
The reviewers on Amazon seem to think this is horror; it certainly is horrible, although horror wasn't the first label I would have thought of.
It was definitely readable, although probably not worth reading. The first chapter was so provocative (Ella implying she was going to die) that I immediately skipped to the last one to find out if she did, before reading the rest of the book.
There are some holes in the tale: What happened to the world outside of the city? It seems more likely than everyone vanishing the world across (although certainly possible), the Overlords merely translated the City into a slightly different reality, leaving everything they didn't want behind. And, what happens when their technology is destroyed?
Not really worth reading, although it wasn't really painful either.