The Louis Sheaffer collection of Eugene O'Neill is an archive of the life and works of Eugene O'Neill formed by Sheaffer's work on his two-volume biography. The success of Sheaffer's biography derives in large part from the extensive research he carried out over some twenty years, and the detailed picture of O'Neill that emerged. All reviews referred to the wealth of information presented in both volumes. The voluminous documentary evidence comprising the Collection fills over 40 archive boxes.

A unique and notable feature of the Collection is the author's typewritten notes from the hundreds of interviews he conducted with individuals who knew O'Neill personally, or knew his relatives, friends or associates. Among the important figures in the O'Neill story with whom Sheaffer became friends and corresponded are the playwright's second wife, Agnes Boulton O'Neill, his third wife, Carlotta Monterey O'Neill, Agnes' daughter Barbara Burton and sister Margery Colman, Lady Oona O'Neill Chaplin (O'Neill's daughter), Jessica Rippin (one of the young O'Neill's New London friends), Beatrice Ashe Maher (O'Neill's most serious New London-era girlfriend), Dorothy Commins (the wife of O'Neill's editor, Saxe Commins), and Cynthia Chapman Stram (Carlotta O'Neill's daughter by an earlier marriage), as well as Mrs. Stram's husband and son.

In addition to 53 of Eugene O'Neill's original letters, the Collection includes several hundred copies or transcripts of important O'Neill letters held by other libraries. There are copies or transcripts of many letters written by Carlotta O'Neill, chiefly to friends, and several from Agnes O'Neill to Eugene. A complete set of Eugene's letters to Beatrice Ashe were copied by Sheaffer from the originals in the Berg Collection of the New York Public Library.

Alphabetical name files contain documents and clippings pertaining to most of the principal figures in the O'Neill story and many of the lesser ones: school classmates, New London-era friends, the Provincetown and Greenwich Village crowd, and those who were personally or professionally associated with him as he rose to the height of his fame in the 1930's. There are extensive clippings files on the plays and a collection of playbills. Several files are devoted to the Provincetown Players and the Theatre Guild.

A picture collection of about 400 prints and negatives includes many formal portraits and informal snapshots of O'Neill at all ages; his wives, children and friends, pictures of the places where they lived, and scenes from productions of his plays.

An extensive O'Neill book collection was formed at Connecticut College before the acquisition of the Sheaffer papers and includes first or limited editions of the plays, scholarly monographs on O'Neill and his works, article offprints, and complete files of the Eugene O'Neill Review and the Eugene O'Neill Newsletter