There's a new biography of Arthur Koestler by Michael Scammell that's just received a lengthy and laudatory review by the estimable Anne Applebaum in the current edition of the NYRB. Koestler's reputation has waned considerably since his death, for a variety of reasons. There's the lack of a manager for his estate (having taken the best qualified candidate with him when he committed suicide), there was the rape incident described in a previous biography by David Cesarani (he apparently didn't take no for an answer with Michael Foot's wife), and, perhaps most significantly, as a Hungarian Jew and native German speaker who wrote in English he doesn't really belong to any one country. And of course the actual suicide itself, where the dying writer took his perfectly healthy 55 year old wife along for the ride, didn't help.
I was an avid reader of everything he wrote when I was in college and I still have considerable respect for his achievements. Darkness at Noon was his masterpiece. Stalin and Russian communism were popular with European intellectuals at the time and Koestler's book helped to remove the blinkers from many. Not Sartre however, who was in denial long after it was reasonable and who broke off relations with Koestler as a result of the book (ok, Koestler also slept with Simone de Beauvoir which didn't help).
The book of his I enjoyed most however was The Sleepwalkers. This was a history of cosmology and astronomy brought to vivid life. I became familiar with the personalities and ideas of the likes of Ptolemy, Tycho Brahe, Kepler, Copernicus and of course Galileo.This was the book that influenced John Banville's wonderful early novels about Kepler and Copernicus. Although a Jew by birth and an early supporter of Zionism he wrote a hugely controversial book, The Thirteenth Tribe, that argued that European Jews are not descended from the Jews who lived in ancient Palestine but rather from the Khazars of Central Asia. A thesis that went down badly in Tel Aviv and New York. Another reason perhaps for his relative obscurity.