Mortal Engines Quartet aka Hungry City Chronicles by Philip Reeve.
From Wikimedia:
"The Quartet is set in the distant future, in an age known as the Traction Era. Earth has been reduced to wasteland by a devastating conflict, known as the
Sixty Minute War. Nations no longer exist, except in the lands of the Anti-Traction League; Traction Cities - entire cities mounted on caterpillar tracks for mobility - are fiercely independent
city-states, using giant jaws to devour one another for resources. Trade is mostly accomplished by
airship, though sometimes cities of roughly equal size (unable to devour each other) will stop to trade. Old-Tech (technology from before the Traction Era, some of which is from the 21st century) is the most sought-after commodity.