A Darkling Plain (Mortal Engines #4)

Agents for peace are bringing an end to the devastating conflict between the roaming Traction Cities and their fanatical enemies while Wren and her father Tom travel the Bird Roads in their airship, trying to forget that Hester has betrayed them. In the ruined wreckage of the city of London they make a discovery that will change the world, while Hester must face an implacable foe with the means and the will to destroy the entire human race.