This book was recommended by Marvin Minsky and referred to in Pinker's How the Mind Works. Minsky cites it as one of the very few scientific analyses of humor, but Koestler's aim was much larger than explaining why jokes are funny. The Act of Creation is a long and dense — but fascinating! — analysis of many different kinds of creativity. Koestler's conjecture is that all creativity stems from what he calls "bisociation" — that is, the joining of two previously separate "matrices" of thought. There's more to it than making candy ("two great tastes that taste great together"), and Koestler's book has changed my view of scientific discovery somewhat. He drives the point home that scientific thought rarely, if ever, consists of gathering lots of evidence and then finding a theory that fits; rather, many of the greatest scientists in history had insights and then searched for evidence to fit their models. Much of scientific discovery is accidental, with advances made based on wholly incorrect understanding of phenomena. Indeed, the main point of the book may be that we are most creative "when rational thought is suspended".

I'm failing to do justice to this book in my description, but I'll add one more note. The section on perception and memory is excellent (and very forward-thinking for the time). Koestler recognized the importance of top-down constraints on perception. He gives a wonderful, extensive example of the auditory perception of a symphony that hit most of the marks in my viewpoint.