Koestler's works somehow form a continuous stream, marking and following the evolution of his thought through the years.
The same themes and reflections can be seen in different phases of development in different works belonging to different genres. This classification is therefore to be considered highly inappropriate: but it was somehow endorsed by Koestler himself, as it is the one followed in Merril & Frazier's bibliography, compiled in 1979 in strict collaboration with the author.
Most of Koestler's works are today out of print, with the exceptions of a few of his novels and essays (Darkness at Noon, The Act of Creation, The Sleepwalkers and The Thirteenth Tribe). The covers of the books come from my collection and are included for purely decorative purposes: they are not for sale!