A darkling plain - Philip Reeve

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.
The fourth,
spectacular instalment of the award-winning quartet.

"No 11-to-16-year-old should miss the superbly imagined debut novel
from Philip Reeve" - The Times

"This big, brave, brilliant book combines a thrilling adventure story
with endless moral conundrums" - The Guardian