If you are looking for one book to read by Koestler, this would be it. "Bricks to Babel" offers extended excerpts, summaries, and explanations of Koestler's most important works sweeping over 50 years of his writing career. It goes from his autobiographical and semi-autobiographical works to his interests in science, philosophy, psychology, religion, and parapsychology. Some of this is, of course, more valuable and viable than other parts. Koestler himself will admit as he progresses through his viewpoints that his beholdenness to Marxist ideology was misplaced and alternatives were needed, as well as his interest in Darwinian evolution which could not explain the hierarchical nature of the universe.
A person could describe the book as Koestler's search for meaning in the universe which ultimately comes to: a hope placed in paranormality. Why? He started with bare materialism which included a foundation in Zionism but found the life hard and relatively fruitless. He switched this out for Marxist materialism only to find that hollow and exploitative. Then he went in a more scientific direction by searching how the material of the universe sorts itself and did achieve some success with the notions of creation/creativity and holonic structure (hierarchies where each part is also a whole of what is below it), but this indicates a higher aspect of the universe driven by mind and teleology. Therefore, Koestler set out to test religion. He already abandoned Judaism, so he tried Christianity but thought it alienated man from God. Then he sought enlightenment in Eastern religions only to find them groundless and disengaged with the material of the universe. So, Koestler sought what went past the established material of the universe in parapsychology, quantum physics, and UFOs. These unknowns gave Koestler enough nebulousness to set his hope for meaning and value on what he could not yet disprove.
Where did Koestler go wrong? Basically, with Christianity. Koestler has an short story called "The Misunderstanding" where he tries to present the thoughts of himself as Jesus going to death at the cross. The title is more than ironic since this showcases how Koestler completely misunderstands Christianity. It is like he fully accepts all historical details of the gospels (including miracles) but fully rejects what everyone (including Jesus) said about these things. He puts Jesus at odds against the God the Father, despite Jesus saying they are one (John 10:31) and everything he does is what the Father has laid out, including Jesus' death on the cross which Christ went to willingly. Koestler misunderstands suffering as God's indifference instead of the evils of this world acting against God's will. Christ went to the cross to suffer that our sufferings might join in his sufferings, so that just as he rose from the dead to enter the glory of God, so will we joined to Christ through faith.