Susan Sontag is prescience personified. Her writings on everything from AIDS to anthropology to psychoanalysis to camp aesthetics can be read today with barely a hint of anachronism. But On Photography perseveres as the most prophetic of the bunch. “Today everything exists to end in a photograph,” she writes in “In Plato’s Cave.” Our era is synonymous with digital self-expression, and Sontag’s 1977 essay collection is a good reminder of the attendant dangers of allowing visual documentation to retroactively define experience.