Servant Mage by Kate Elliott
An absolute gem of a story... I loved it. -S. A. Chakraborty They choose their laws to secure their power. Fellion is a Lamplighter, able to provide illumination through magic. A group of rebel Monarchists free her from indentured servitude and take her on a journey to rescue trapped compatriots from an underground complex of mines. Along the way they get caught up in a conspiracy to kill the latest royal child and wipe out the Monarchist movement for good. But Fellian has more than just her Lamplighting skills up her sleeve.