I think that 20th Century literature has been influenced by the thinking and writing of Karl Marx.  Marxist thought in terms of the shifting relationships between individuals and wealth has helped to construct some of the strongest understandings of literature and literary criticism.  The relationship between wealth and power, in terms of how social attachments are formed and dissolved has helped to construct some of the best novels of the century.  Steinbeck's works like The Grapes of Wrath and Of Mice and Men are built upon the Marxist questioning of the capitalist system.  Faulkner's The Sound and the Fury as well as Morrison's Beloved  can both be seen in the light of Marxism and the relationship between those who have wealth and power and those who lack it.  Marx's ideas are also in play with the works of F. Scott Fitzgerald in The Great Gatsby and in Arthur Miller's Death of a Salesman.  The structure of wealth and capital acquisition and how it impacts characters in literary narratives are themes that Marx harvests in the 19th century and literature in the 20th century explores.