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. Also 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.

Additional 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.