In the philosophic essay The Birth of Tragedy (1872) Friedrich Nietzsche outlines his vision of the tensions between the Apollonian and Dio-nysian impulses and discusses the uses of masks in Greek tragedy (paperback editions from Dover and Oxford University Press).

August Strindberg's surrealistic A Dream Play, which opened in 1902, became the forerunner of modern expressionism and influenced a new generation of dramatists, including Eugene O'Neill (widely available).

A collection of Sigmund Freud's work can be found in The Ego and the Id (The Standard Edition of the Complete Psychological Works of Sigmund Freud). This volume presents Freud's theories of the subconscious (in paperback from W. W. Norton, 1990).

Long Day's Journey into Night, first performed in 1956, is O'Neill's finest study of domestic interaction and offers insight into O'Neill's own tragic relationship with his family (widely available).