Mortal Engines by Reeve

In the far-distant future, the world is decimated in the 60-minute war and cities travel on rolling tracks, sort of like enormous tanks. The cities hunt other, smaller towns and use their resources.

Tom is an apprentice historian in London and helps sort through the old things found on eaten towns. His hero is Valentine, the head historian. When a girl from an eaten town tries to kill Valentine, Tom comes to the rescue. Unfortunately, Valentine is not what he seems so both Tom and the would-be assassin Hester are thrown of the city and left for dead. The pair have to reconcile their ideological differences and work together to get back to London and save the world.