Are any of these writers on socialy relevant writing or for 'art for art's sake' and why?

Theodore Dreiser was socially relevant. His books "An American Tragedy" and "Sister Carrie" reveal the darker side of human existence. Also Emile Zola, a French writer, was also socially relevant. His book "Nana" like Dreiser's books, reveals the dark underbelly of a society gone industrial.