This series consists of personal and work-related correspondence (primarily incoming correspondence) between Cahill and various friends and colleagues. While a large portion of the series documents Cahill's position as Director of the FAP, it also extends beyond those years and illuminates other aspects of Cahill's career including his interest in folk and Asian art, and his work as an art critic.

There is significant correspondence with the artist Stanton MacDonald Wright between 1936 and 1950, and with the artist Irene Pereira between 1950 and 1953. The series also documents research which Cahill conducted in the late 1940s on the development of the Index of American Design for his introduction to a book on the Index by the National Gallery of Art, published by the Macmillan Company. Correspondence from 1949 provides another angle on the historical details of the FAP through lengthy correspondence documenting Cahill's criticism of William Francis McDonald's book (Ohio State University Press, 1969).

There is a large amount of correspondence from July 1960 comprising sympathy letters to Dorothy C. Miller following Cahill's death. Correspondence from 1977 encloses a catalog of an exhibition organized by New York WPA Artists, Inc., at the Parsons School of Design in November 1977. The exhibition, , was dedicated to the memory of Holger Cahill.

A list of most of the correspondents (with the exception of those microfilmed on reel 1105) can be found in the Appendix: Alphabetical List of Correspondents in Series 2