The Lives of the Caesars. by Suetonius

Suetonius was a Roman historian born in about 69 AD, shortly after the death of the emperor Nero. This book, detailing the lives of the twelve Roman emperors who were known as "Caesar"-some by a family connection to Julius Caesar, some just as a title-is considered to be Suetonius' most important work. The Lives of the Caesars is a detailed account of the often dramatic lives of these emperors, whose abilities and morals varied enormously ; from the capable, stable Augustus, to the insane Caligula.
Several of these men died violently either by their own hand or by assassins. Suetonius, though, is careful to give credit where it is due, outlining the better actions and laws of each alongside an account of the crimes and immoralities they also carried out. This turbulent period of Roman history has often been depicted in fiction and in media, drawing on the work of Suetonius and other contemporary historians.
For example, Robert Graves' novel I, Claudius (1934), which was made into a highly-controversial television series by the BBC in 1976.