The Act of Creation
While likely a less-than-reputable person given his influences and acquaintances, it can not be denied that this, like many of his other works, are at a minimum thought provoking. One of the main concepts explored in the book is bisociation; connecting two unrelated frames into a novel concept. These frames can be learned in any traditional manner (rote, etc) or conditioned into a subject a la Pavlov, Skinner, or Watson's experiments. Either way, to move from one frame to another, solving a problem usually, one needs creativity and originality more than genius (though having both is obviously best). Genius, Koestler and other argue, is a mind with general power toward in a particular direction and the ability to make bisociations.
Introducing Evolutionary Psychology
An evolutionary look at psychology assuming that the mind is made up of modules that have evolved independently and in different environments. This view follows the older behaviorist belief that actions were caused, not by the mind, but by environmental stimulation. A core of evolutionary psychology is to look at the mind's computer modules made for processing information. Some of these modules are supposed to lead to certain innately human abilities including language, physical attractiveness, and food preferences that seem to span all cultures. The conclusion is that our minds have evolved in an environment that is totally foreign to our current environment; we are essentially monkeys living in the fast lane.