One day, nonagenarian Gordon S. McCulloch was passing the days with his pony in a small English village, penning poems that only his friends and family would read. The next, his self-published collection of verse, “101 Poems,” was at the top of Amazon’s sales charts and bedecked with five-star reviews from all over the world. Thanks to a viral tweet from his granddaughter, the Scottish retiree was a bestselling poet at 96.