The archive consists of inscribed and annotated books from Koestler's library, along with English-language editions and translations of his work into many languages; and Koestler's manuscripts and papers from 1940 to March 1983. The archive contains correspondence with or about members of Koestler's family; personal material including diaries, address books, medical files and other personal documents; literary manuscripts including interviews, broadcasts and speeches; correspondence with specific people; personal and fan correspondence; material on specific subjects such as ESP and euthanasia; business and financial papers; literary manuscripts by other authors, including those on Koestler; cuttings, offprints, and pamphlets including articles both by Koestler and on Koestler; non-print material such as photographs, tapes and medals, and records; and, files gifted by publishers Hutchinson comprising of papers and correspondence on particular books. Most of Koestler's earlier papers were lost when France fell in 1940 and when he left for England. The papers bequeathed in 1993 include personal files which are restricted until 2045; correspondence; reviews; royalty statements 1976-1993; contracts and rights correspondence; personal financial material; and, correspondence on wills, last message, sale of property, and winding up the estate.
In addition there is a greetings card to Kenneth H.S. Cohen (1900-1984), Intelligence Officer and industrialist, and Mary S. Cohen (1910-1991), artist, from Arthur Koestler, Christmas 1962 at E2008.29.