The History of the Decline and Fall of the Roman Empire, Vol. 1

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. 1

Diligence and accuracy are the only merits which an historical writer may ascribe to himself; if any merit, indeed, can be assumed from the performance of an indispensable duty. I may therefore be allowed to say, that I have carefully examined all the original materials that could illustrate the subject which I had undertaken to treat. Should I ever complete the extensive design which has been sketched out in the Preface, I might perhaps conclude it with a critical account of the authors consulted during the progress of the whole work; and however such an attempt might incur the censure of ostentation, I am persuaded that it would be susceptible of entertainment, as well as information.