Singer-actress Jeanette MacDonald is a perfect example of what, decades after her death, became known as a "classical crossover" artist. In her films, radio, television appearances, concerts, and recordings, she sang opera, operetta, art songs, and show tunes, often with an eye toward popularizing classical music for the masses. This was a delicate balancing act that worked for most of her career, during which she enjoyed high-grossing films, gold records, and sold-out concerts. But even during that period, like all classical crossover artists, she suffered criticism from both...