The History of Rome by Theodor Mommsen

Mommsens history of the Roman Republic in five volumes. These volumes cover from the founding of Rome up to the disintegration of the First Triumvirate (Caesar, Pompey, Crassus). The author cites dates in the Roman style, AUC, from the founding of the city in 753 BC. A table at the end shows modern-day equivalents. According to Wikipedia, Mommsen is generally regarded as the greatest classicist of the 19th century. This work complements Gibbons Decline and Fall of the Roman Empire.