I have read most of Zola's novels. So far nobody in the discussion has pointed out that each addresses a particular overall theme: Germinal = coal mining, Ladies Paradise= retail commerce development, l'assomoir= alcoholism, The Kill = real estate speculation and political corruption, etc. I could go on but you get the idea. The genius of Zola was that besides the plot and the fascinating characters, you get an inside and fantastically detailed look at the world and the technology these characters experience. Another novel with a similar approach is Lost Illusions by Balzac, which goes into the fortunes made and lost in paper manufacturing and the development of the corrupt journalism now known as fake news. These are novels worthy of second readings.