The House of Storms — Ian R. MacLeod

To save her only son from the ravages of consumption, the Greatgrandmistress of the Telegraphers Guild, Alice Meynell, has made a bargain with her former lover-a changeling in the land of Einfell where magic may accomplish what science could not-to do whatever is necessary to cure him.

Once he is healthy again, her son will be torn between his duty to lead the guild and his love for a servant girl. But Alice has already chosen her son's destiny and she'll see him achieve it even if it means ending the Age of Light.