Reed Merril & Thomas Frazier: Arthur Koestler - An international bibliography
(Ann Arbor: Ardis 1979)

This lengthy bibliography of Koestler's works is a useful resource for anyone researching Koestler at depth. It lists over 1.000 entries, divided in the following categories:

Works by Arthur Koestler
    Fiction and Drama
    Essays and Symposia
  Contribution to books
  Contribution to periodicals
    Book Reviews
    Notes and Correspondence
  Symposia, Lectures and Broadcasts
Works about Arthur Koestler
  Books and Dissertations
  Articles in Periodicals and Books
  Articles in Periodicals and Books: Individual Works
Recent Science Literature Citing Works of Arthur Koestler

Care must be paid, nevertheless, to the numerous mistakes made in the references, mistakes that turn out rather evident after shortly browsing through the book.