Steam Mountain — Sue Perkins

Maevens Ash and Orm's continued search for Ash's parents bring them to the country of Stavmo, a civilization whose power is based on steam.

Steam cars, whirlybirds, and small appliances, even guns, are all powered by steam... but at a cost. A huge factory has toxic smoke billowing into the air, and only the rich are able to escape the smell by living on the opposite side of town. Class is based on your hair color, and if your hair isn't red like the dictator who runs the country, you vanish into the mines or are thrown into the volcano.

But the volcano holds even greater secrets. And possibly a link to Ash's family.

The magical pair make friends with the rebels and try to help them fight for their rights, but in doing so they get separated and place themselves in danger.