I have not read any books by Emile Zola, but have heard him mentioned as the writer who influenced Steinbeck and others. Zola was the leading writer of Naturalism in the 19th century and he did meticulous research before writing a novel. His masterpiece is the novel Germinal about a miner's strike in France.
Somewhere I read this bit of trivia. While researching for the novel, Zola went into a coal pit to actually understand the working of a mine. He saw a horse pulling the coal carts and was surprised. He asked the workers how the big horse came down the tunnel. The workers said, "You really don't know much about the mines. The horse is brought down the tunnel when it is young and small. It grows up in the pit, does the work and is buried here on death. It never goes out again, never sees the sunlight". This shook Zola so much that he made the horse an important character in the novel.
The imagery has stuck with me. A disturbing one, reflecting our lives.