This series contains letterpress copy books of letters sent by Cyrus H. McCormick, Mrs. Nettie Fowler McCormick, Cyrus H. McCormick Jr., and the McCormick's personal and business secretaries. Letters pertain mainly to the affairs of the McCornick Harvesting Machine Company, but also relate to the McCormick Theological Seminary, donations, checks withdrawn and their disposal, politics, mines, personal and social matters, and patent litigation.

Though the volumes are numbered 1 to 24, an original “letter” designation has also been retained due to references to these letter designations in the correspondence of Cyrus H. McCormick.

Most of the volumes are indexed alphabetically according to the name of the addressee and most of the letters will appear in the main catalogue of the Cyrus H. McCormick Papers.