Coincidence, synchronicity, and paranormal events of varying types have and are lumped together with dismissive shrugs by many, but not all, learned people in a host of disciplines. That Arthur Koestler turned his keen intellect on these topics is not as surprising as some of the postulates he made, many of which stand still as neither disproved or confirmed.
He chronicles much of the research done to confirm E.S.P. and discusses both the flaws and results, results that tend to favor the existence of psychic abilities despite the fact that rigid scientific methods demotivate participants and suppress such abilities. Mr. Koestler makes the point repeatedly that coincidence, synchronicity, and parapsychology cannot necessarily be bottled up or tapped on demand, but that sometimes precipitating events will trigger such events. (I've experienced such things first-hand and concur with this view.)
Mr. Koestler---like other divergent thinkers such as Robert Anton Wilson, Fritjof Capra, Karl Pilbram and others---turned to physics for explanations for these phenomena, believing that an intrinsic link must exist between these two seemingly disparate disciplines. His writing on this subject is fairly strait-forward, though because this book is more than 30 years old, much of what was then state of the art now seems quaintly misdirected (sort of like the depiction of the giant HAL artificial intelligence in "2001: A Space Odyssey").
An essential point, for me, are that Mr. Koestler takes great pains to show that learned, prestigious persons---e.g., Jung, Rhine, Kammerer, Eddington---have been delving into this research for centuries, often at great personal cost to their careers or images.
Rereading this book now, I tend to side with Jung's view of refusing "to commit the fashionable stupidity of regarding everything I cannot explain as a fraud." In other words, I'm not sold on everything here, but I think this well-articulated treatise on parapsychology that well-worth the time. Or put another way (with apologies to Lennon): All we are saying is give E.S.P. a chance.