A Darkling Plain (Mortal Engines #4) by Philip Reeve

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.