Le Ventre de Paris by Émile Zola (1873) is not set in Saint-Étienne, but Zola's novels of the Second Empire — the Rougon-Macquart cycle — are essential background for understanding the industrial France that made cities like this one. Germinal (1885), his novel of a northern coalfield, is the direct equivalent: miners, strikes, solidarity, and the grinding weight of industrial capitalism. Saint-Étienne's mines closed a century after Zola wrote about them, but the social world he described persisted here long after it ended elsewhere.
Les Misérables by Victor Hugo (1862) is set partly in the industrial regions of northern France and touches on the lives of workers and the conditions that shaped French radical politics. Hugo understood the Loire basin as well as he understood Paris, and the novel's social panorama helps explain why cities like Saint-Étienne consistently voted left for a century.
Stèles by Victor Segalen (1912) is not about Saint-Étienne at all — it is a sequence of prose poems modelled on Chinese stone inscriptions — but Segalen was one of the most important French writers to come from the industrial south, and his sense of place and material culture has something of Saint-Étienne's rougher, more honest aesthetic in it.