Stranger on the Square is the third volume of Arthur Koestler's autobiography, published posthumously in 1984. It was co-authored with his wife Cynthia Koestler, née Jefferies, and includes autobiographical notes of her as well. The book was published by Hutchinson, London 1984, 242 pages including Index. ISBN 0-09-154330-4. It was edited by Harold Harris, who wrote the Introduction and Epilogue. He was Koestler's editor for many years. The book is illustrated with seven monochrome photographs plus a colour and another momochrome photograph on the dustcover.
The structure of the book is unusual. Part One 1940-1951, has six chapters whose authors are Arthur Koestler and Cynthia Koestler, alternately. Part Two 1951-1956, also consists of six chapters, all by Cynthia Koestler, followed by an Epilogue written by Harold Harris, an Appendix by Arthur Koestler, and an Index.