Infernal Devices by Philip Reeve

For years, Tom Natsworthy and Hester have lived in relative happiness in Anchorage. The city has been hiding in the unexplored regions away from the predators. But all that is about to change. The Lost Boys have found the city and are looking for something. Tom's daughter Wren has been raised on stories of the Lost Boys and traction cities and she is so excited to meet one of the boys in person. He doesn't seem so dangerous. And all he wants to steal is one book from the library. Wren is so tired of living in Anchorage and longs to travel and have adventures. And she is really tired of her mother. Hester always seems to put her down. So, when the Lost Boys promise to take Wren with them, she eagerly steals the book and delivers it to them. But what will she find out about their true nature. And the contents of the book?