These indexes contain individual, corporate, and subject entries, which include descriptive notations of letters received and sent concerning civil engineering, the Pennsylvania Railroad, and personal business (horses, racing, billings, meetings). Volume One includes notations of exhibits of the paintings by his sister Mary Cassatt (p. 269). There are also notations on paintings by Thomas Eakins, John Singer Sargent, and James Whistler.

The collection of Charles Coleman Sellers (1903-1980) contains copious and detailed documentation of the art of Charles Willson Peale and his family. It consists of working files for Sellers's numerous publications, including his Portraits and Miniatures by Charles Willson Peale (1952); Charles Willson Peale with Patron and Populace (1969); C. W. Peale's Portraits of Washington (1951); Benjamin Franklin in Portraiture (1962); Mr. Peale's Museum (1980). Most files include photographs of the art work, notes on the piece, and correspondence with authorities or owners. Other series include one relating to the paintings of various other Peales, including Anna C., James, Mary Jane, Raphaelle, Rembrandt, Rubens, and Sarah Miriam, and a miscellaneous artist file, which includes the same type of material and information on many eighteenth- and nineteenth-century artists, including Thomas Eakins, George Healy, Robert Edge Pine, William Rush, Thomas Sully, Benjamin West, James Abbott McNeill Whistler, etc. There is a separate Sellers collection at Dickinson College, primarily personal in nature.