The papers of William Appleton Aiken consist of manuscripts, transcripts and photostatic copies of papers in British and American repositories, notes, correspondence, photographs, and other papers relating to Daniel Finch, second Earl of Nottingham, and his period. The bulk of the collection is research materials for Aiken's Yale doctoral thesis, The Conduct of the Rt. Hon. Daniel Earl of Nottingham, By Himself (1939; published in 1941 under the title, The Conduct of the Earl of Nottingham, and for other writings on related topics. The research materials have been left in Aiken's order.
Aiken's manuscripts are largely critical editions of writings by Nottingham and his contemporaries, although several short articles are also included. His correspondence consists of letters from Jasmine Finch about her family's history, from Aiken's research assistant, A. May Osler, and from historians and scholars sending congratulations on the publication of his book. Included are Stuart W. Jackson, Wallace Notestein, Paul D. Moody, and George M. Trevelyan.
Also in the collection, although unrelated to the other materials, are two autograph letters of 1897, one from Cosima Wagner and the other from Hedwig Humperdinck, both addressed to Charles Joseph Dyer.
Also included are Aiken's personal papers including his diaries as an army intelligence officer in the Middle East during World War II and an exhibition scrapbook prepared by the Overseas Branch of the Office of War Information.