Thomas Gainsborough (1727-1788), admired for his grand society portraits and sumptuous pastoral landscapes, is the most perennially popular of British artists. In his life as in his art, Gainsborough sought to project an image of effortless accomplishment, demonstrated by a dazzling painting technique and immense personal charm. He was also competitive, opinionated, and financially astute. Because he was among the most innovative and enigmatic artists...