Charles Spencer, the brother of the late Princess Diana, has made headlines recently with the claim that King Charles III said she, his ex-wife, would soon be forgotten after her death in a 1997 car accident in Paris. In his book Swan Song: Diana, My Sister, due to be published next week, Spencer claims then-Prince Charles told him: “Rest assured, we’ll forget her soon enough.” He also accuses Charles of having “contempt” for his ex-wife and of sounding “elated” after her death. While...