A Darkling Plain by Philip Reeve

The fourth and final book in Philip Reeve's Mortal Engines Quartet.

The great Traction City of London now lies in ruins. Once, not so long ago, it hunted other cities across the vast wastes of a devastated land. Now it is a poisonous, lifeless wreck - until Tom Natsworthy and his daughter, Wren, uncover the secret hidden in its dark heart. It's a secret that could bring an end to the war once and for all. But as Tom and Wren risk their lives, time is running out. Far away, a dangerous child with a deadly secret of his own is making a journey that may lead to the destruction of the entire human race.