According to Wikipedia, " The Twelve Caesars is a set of biographies of twelve successive Roman rulers, from Julius Caesar until Domitian, entitled De Vita Caesarum...It was written by Gaius Suetonius Tranquillus (ca. 69/75 after 130). The Twelve Caesars is considered very significant in antiquity and remains a primary source on Roman history. The book discusses the significant and critical period of the Principate from the end of the Republic to the reign of Domitian. The work, written in 121 during the reign of the emperor Hadrian, was the most popular work of Suetonius and is the largest among his surviving writings... Gaius Suetonius Tranquillus, commonly known as Suetonius (ca. 69/75 after 130), was an equestrian and a historian during the Roman Empire. His most important surviving work is a set of biographies of twelve successive Roman rulers, from Julius Caesar until Domitian, entitled De Vita Caesarum. Other works by Suetonius concern the daily life of Rome, politics, oratory, and the lives of famous writers, including poets, historians, and grammarians. A few of these books have partially survived, but many are entirely lost."