“Rome is the most glorious place in the Universal World”—this was how the twenty-six-year-old Scottish architect Robert Adam described his reaction to the city on his arrival in 1755. Both Art in Rome in the Eighteenth Century and the exhibition it was created to accompany are lavish, vivid demonstrations of that assertion. The catalogue, however, is much more; it combines illustration of the exhibition—called The Splendor of 18th-Century Rome and held at the Philadelphia Museum...