Diaries, correspondence, letterbooks, writings, lectures, family papers, financial records, legal file, scrapbooks, and other papers relating to Ingersoll's involvement in Illinois state and Republican Party politics; lectures and writings on agnosticism and religion; law practices in Peoria, Ill., and Washington, D.C.; and personal and family affairs. Other topics include anti-vivisection, the gold standard, impeachment of Andrew Johnson, Reconstruction, tariffs, and women's suffrage. Includes Ingersoll's address before the 1876 Republican Convention in Cincinnati, Ohio. Family correspondents include Ingersoll's wife, Eva Amelia Parker Ingersoll; their daughters, Eva Ingersoll Brown and Maud Ingersoll Probasco; his brother, Ebon C. Ingersoll; Clinton Pinckney Farrell; and Sue Parker Farrell.

Correspondents include James Gillespie Blaine, Harriot Stanton Blatch, Edward William Bok, John Burroughs, Benjamin F. Butler, Andrew Carnegie, Moncure Daniel Conway, Eugene V. Debs, Thomas Dixon, Edgar Fawcett, Henry M. Field, Minnie Maddern Fiske, Melville Weston Fuller, Walter Quintin Gresham, John Marshall Harlan, Rutherford Birchard Hayes, George Jacob Holyoake, John E. Mulholland, Richard J. Oglesby, Max O'Rell (Paul Blouët), Courtlandt Palmer, Parker Pillsbury, James Redpath, Thomas B. Reed, Eduard Reményi, Anton Seidl, Elizabeth Cady Stanton, Horace Traubel, Mark Twain (Samuel Langhorne Clemens), and Walt Whitman.