It was a warm April in Pleasant Valley when Margaret Korda, normally a fearless horsewoman, dropped her horsewhip while she was riding. Such a mild slip was easy to ignore, but when other troubling symptoms accumulated, she confided to her husband, "Michael, I think something serious is wrong with me." Within a few rapid weeks, the fiercely independent, former fashion model was diagnosed with brain cancer, while Michael, once reliant on her steeliness,...