Scrivener's moon - Philip Reeve

In a future land once known as Britain, nomad tribes are preparing
to fight a terrifying enemy - the first-ever mobile city. Before
London can launch itself, young engineer Fever Crumb must journey
to the wastelands of the North. She seeks the ancient birthplace
of the Scriven mutants. In the chaotic weeks before battle begins,
Fever finds a mysterious black pyramid. The extraordinary secrets
it contains will change her world forever. The seventh awe-inspiring
adventure in the World of Mortal Engines series by a superb
writer at the height of his powers.