The British have always been good at silence — at family meals spent wordlessly; intense emotions expressed through a hand on the shoulder — but on Sept. 6, 1997, they surpassed themselves. London, the big, braying capital, was stilled as over a million mourners of Diana, Princess of Wales, kept vigil along the route to Westminster Abbey. The hush amplified the sounds of the cortège as it set out from Kensington Palace: the rumble of wheels on tarmac, the clopping of horses' hooves, and a bell that tolled at listless intervals. But as the procession came into view, turning out of...