If it weren’t for the black death, the fourteenth-century merchant Francesco di Marco Datini might have left no trace in history. A workaholic Tuscan with offices from Bruges to Barcelona, he spent most of his time scribbling letters, often boasting that he slept only four hours a night. He kept his wife busy distilling vinegar, entertaining visitors, and sewing helmets for the arms trade; Datini himself, meanwhile, was married to the game of late-medieval commerce. At the time, it was considered a perilously sinful hustle. One friend warned the merchant that he was “a man who kept women and…