I've just finished Michael Scammell's surely definitive biography of Arthur Koestler, one of the great intellectual heroes of the 20th Century. It's a warts and all job with particular emphasis on Koestler's relentless womanising. He liked to operate from a domestic base so he was always living with one woman while he chased the others. But he was as relentless in the pursuit of truth and political justice as he was in chasing a well-turned ankle. It's remarkable the number of great events in European history he was involved in. He was an early Zionist and travelled to Palestine in the early 1920's to support the burgeoning Israel (but found the physical labour on a kibbuz not to his taste); he was Spain during the Spanish Civil War (narrowly escaping execution) and was in France when the Nazis invaded.
He saw through Stalin long before most of European intellectuals (such as Sartre) and his most famous novel (Darkness at Noon) did much to open people's eyes to the monster in the East. His campaign against capital punishment in England led to its eventual abolition, his articles in the Observer and his book Reflections on Hanging were hugely influential. Eventually he became disillusioned with politics and turned his attention to science. My favourite book of his is The Sleepwalkers, an accessible study of the history of cosmology that brought you into the worlds of Kepler, Galileo, Copernicus and Tycho Brae. A book I'm sure John Banville read before he started his novels on Kepler and Copernicus. I also enjoyed The Act of Creation, a study of the origins of creativity. Late in life he got into trouble with the Jewish lobby for having the temerity to suggest that the European Jews that emigrated to Palestine may have descended from the Khazars rather than from one of the 12 tribes of Israel. He also dabbled with parapsychology - an interest that did much to diminish his later reputation.
When his health deteriorated (he had Parkinson's amongst other ailments) he committed suicide along with his much younger (and healthier) wife Cynthia. He got much abuse for apparently dragging her into it but anyone reading of her long-term devotion to him would not have surprised at her willingness to join him.