The History of the Decline and Fall of the Roman Empire, Vol. 3

Edward Gibbon's massive and masterful six volume work, The History of the Decline and Fall of the Roman Empire, is a hefty undertaking. But it's cited as the model on which modern historians base their inquiries into the past. The books cover Roman history from 180 AD to 1590, a period from Marcus Aurelius to a time not too distant from Gibbon's own. 

The author spent a good part of his life working on Decline and Fall, and as each volume was undertaken he compared it to the birth of a child. His theory is not without criticism, but in general the causes of the Fall were the weakening of civic "Roman" virtues (such as their military spirit), the apathy that Christianity bred in its culture, and the eventual invasion of barbarian hordes that had become a fixture of Rome and its territories. Whatever the legacy of his work, Gibbon's use of primary sources over secondary texts cemented his fame as the "first modern historian."

Preface of The History of the Decline and Fall of the Roman Empire, Vol. 3 

The fame of Gratian, before he had accomplished the twentieth year of his age, was equal to that of the most celebrated princes. His gentle and amiable disposition endeared him to his private friends, the graceful affability of his manners engaged the affection of the people: the men of letters, who enjoyed the liberality, acknowledged the taste and eloquence, of their sovereign; his valor and dexterity in arms were equally applauded by the soldiers; and the clergy considered the humble piety of Gratian as the first and most useful of his virtues.