The Appleton-Century mss., 1846-1962, consists of the office files of the publishing company, its two predecessors, D. Appleton & Co., and the Century Co., and to a small extent its successor, Appleton-Century Crofts, Inc.
The Armour mss., 1952-1956, consists of letters from author Upton Sinclair, 1878-1968, to author and English professor Richard Willard Armour, 1906-1989.
The Boni mss., 1926-1939, consists of the correspondence of author Upton Beall Sinclair, 1878-1968, with publishers Albert Boni, 1892-1981, and Charles Boni, 1894-1969.
The Cane mss., 1936-1957, consist of letters from Upton Sinclair, 1878-1968, author, to Melville Henry Cane, 1879-, lawyer and poet. They relate in the main to Sinclair's writings.
The Clark mss., 1909-1957, consists of the letters and writings of author, editor, and publisher Thomas Curtis Clark, 1877-1953, as well as the autographs and poems of other writers collected by him.
The Colorado-Sinclair mss. consists of letters exchanged between author, Upton Beall Sinclair, 1878-1968, and University of Colorado Director of Libraries Eugene Holt Wilson, 1909-?, Boulder, Colorado, August 25-October 31, 1950.