Koestler seeks to demolish the assumption that the history of this particular subset of science is a Whiggish story of Progress towards a Theory of Everything. Rather it is a story of various people blundering around and gradually and haphazardly building a picture of the universe.
These philosophers and scientists "sleepwalk" towards understanding.
Enjoyable and iconoclastic, well written but with some flaws.
Some spiritualist stuff, which comes off sounding a little odd against the otherwise respectably skeptical tone.
Claims that electromagnetism and gravity are "verbal fetishes ... disguising the fact that they are metaphysical concepts dressed in the mathematical language of physics."
Well that's an interesting perspective. I wish I knew if Koestler was speaking from a view of ignorance or understanding as regards the mathematical underpinnings of gravitational theory.
It is unfair to say gravity is a metaphysical construct. It is a theory that currently suits all available evidence very well. We believe in it because the evidence suggests it exists.
However Koestler is right to reinforce how peculiar the idea of "action at a distance" (spooky or otherwise) really is. Descartes was having none of it. Newton presented his theory of gravity but said:
"gravity must be caused by an agent acting constantly according to certain laws; but whether this agent be material or immaterial, I have left to the consideration of my readers"
Dismissive of Einstein (p 504, Chapter 3 The Newtonian Synthesis 1. 'Tis all in Pieces):
"Einstein's correction to Newton's formula of gravity is so small that for the time being it only concerns the specialist. The two most important branches of modern physics , relativity and quantum mechanics, have not so far been integrated into a new universal synthesis; and the cosmological implications of Einstein's theory are still fluid and controversial."
I think Koestler rather misses the point here: as I understand it Newton was wrong to assume space is "flat" and Einstein demonstrates that space is not flat. Newton's theories are a good approximation of what goes on in the universe, and for most purposes do well enough, but Einstein is closer to being correct than Newton.
The book was published in 1959: I think Einstein's theories were bedded down by then.
All in all, an excellent read, drawing on the original sources.