Mortal Engines by Philip Reeve

London is hunting its prey. The great Traction City lumbers after a small town, eager to strip its prey of all assets and move on. Resources on the Great Hunting Ground that once was Europe are so limited that mobile cities must consume one another to survive, a practice known as Municipal Darwinism. Tom, an apprentice in the Guild of Historians, saves his hero, Head Historian Thaddeus Valentine, from a murder attempt by the mysterious Hester Shaw only to find himself thrown from the city and stranded with Hester in the Out Country. As they struggle to follow the tracks of the city, the sinister plans of London’s leaders begin to unfold.