Stoneheart (Stoneheart Trilogy, Book 1‪)‬

A city has many lives and layers. London has more than most. Not all the layers are underground, and not all the lives belong to the living. A twelve-year-old boy named George Chapman is about to find this out the hard way. On a school trip he's punished for something he didn't do. In a tiny act of rebellion, he lashes out at a small carving on the wall - unexpectedly breaking it off. And then something horrible does happen: a stone Pterodactyl unpeels form the wall and starts chasing him. George is already running before his mind starts trying to tell him this is impossible.