Last year on March 3rd the bodies of Arthur and Cynthia Koestler were found in their house in Montpelier Square. Koestler’s membership of Exit was well known. His belief in voluntary euthanasia can be traced back to the way he overcame the terror of an operation in childhood. He described in Arrow in the Blue how he insisted on holding the ether mask over his own face. ‘In this way I would feel that I was in control of the situation, and that the terrible moment of helplessness would not recur.’

It was the double suicide which affected the imagination, but in all the articles at the time I saw no reference to either von Kleist’s suicide pact with Henriette Vogel, or to that of Stefan Zweig with his wife in Brazil in 1942. Although Zweig and Koestler were both Jews born in the Austro-Hungarian Empire, who died British citizens, they could hardly have been more different. And yet there was a link which had intrigued me even before Koestler’s death: whether his theory of man going to war through fear of his peer group’s opinion was inspired by Zweig’s Beware of Pity, or whether a common source in Vienna had influenced them both.

The background to Cynthia Koestler’s decision to die with her husband becomes clear in this remarkable and unexpected book. I have always loathed the cliché ‘reads like a novel’ but the phrase kept intruding, not just because it was a pleasure to read. Koestler’s character and Cynthia’s obsession provide obvious