This is the third novel in the Rougon-Macquart cycle, and is about the food market of Les Halles, in the centre of Paris. It was built in 1851, not long before the action of the novel, in 1858-9, and, like Covent Garden in London, it no longer exists on its original site, having relocated for similar reasons.
The plot of the book is quite thin - it involves Florent Quenu, recently returned to Paris after escaping his imprisonment overseas for alleged involvement in the coup of 1851. He finds and lodges with his younger brother, and becomes an inspector in the market. There are various rivalries amongst the stallholders for his affections, although he barely notices them being more wrapped up with planning an insurrection in revenge for his deportation. He is inept and indiscreet, however, and is arrested and deported once more.
The rest of the novel, probably half of it, is taken up with Zola's descriptions of the market and its operation. There are multiple page inventories of the stocks of food sold, which is all very nice but serves little purpose except as documentary. This is one of Zola's failings - he did lots of research for his books, and needed to display it, but that can inhibit a narrative, and does here.
There is a small amount of political philosophy - the theory of the Fat and the Thin, expounded by the painter Claude Lantier, who is the central characteer of the later L'Oeuvre. The Fat are the forces of conservatism and complacency, the Thin are the reformers. It's quite simple, and not greatly illuminating.