No one ever throws away a letter, or even a postcard. We delete emails, we throw away old tax returns, yellowed copies of divorce decrees and lawsuits. But we keep our letters and postcards. We keep them in folders, in boxes in the garage and attic. Correspondence comprises unspoken personal and family history. Decades, generations, centuries often pass before these letters become part of the weave of our lives, part of what we, and our ancestors,...