In the view of many, O'Neill's A Long Day's Journey into Night (first produced in 1956) is his greatest play. Based on the troubles of O'Neill's own family, the play follows the Tyrone family through one day in their tormented lives. Family background is revealed as they argue with each other, and in the end they retreat into the solace provided by alcohol and morphine.

Eugene O'Neill: Beyond Mourning and Tragedy (2002), by Stephen A. Black, is a critical biography that examines O'Neill's life and work in the light of Freudian psychoanalysis. Black argues that O'Neill's plays came out of his attempts to understand his difficult relationships with his parents and his brother, all of whom died within a three-year period. According to Black, O'Neill mourned them for twenty years in his plays.

Louis Sheaffer's two-volume biography, O'Neill: Son and Playwright (1968) and...