This series contains the correspondence of the editors with the contributing authors, advisers and potential contributors. Most of the correspondence is in English, but European correspondents commonly employ the French and German. The list of correspondents below is not exhaustive, but includes only major sets and famous names: the series as a whole contains correspondence with more than 1200 persons. Much of the correspondence with famous people, such as W.E.B. DuBois and Oliver Wendell Holmes, consists merely of polite refusals, but many, such as Margaret Mead and Felix Frankfurter, did make substantial contributions to the Encyclopaedia.
A large gap exists in this series: there are no files from correspondents with names beginning with the letter C; in fact, none between the letters BR and DR. This material was not present when the collection was first inventoried in 1984 and must have been lost prior to that time.