For the first decade of the rock 'n' roll era, Elvis was unrivaled in fame, fortune, and importance. He was a sensation with virtually no precedent, but following a stint in the Army just when he was peaking, the focus of his career shifted from making dangerous music to mass-producing safe movies. Then came the TV event that we now know as the “’68 Comeback Special.” It was originally supposed to be an hour-long ode to Christmas. What it became instead was a showcase for the power of...