Charles C. Mann has faced up to the locavore’s dilemma. At his home in the Berkshires, he likes to eat food that has traveled directly from his own garden: heirloom tomatoes, eggplant, bell peppers, kale, chard, lettuce and other foods for his table. He and his family belong to a farm-share program in which they advance money each year to a farmer a few miles away in return for the farm’s crops. He loves local food, but he knows too much about it to be a truly devout locavore.