Edward Gibbon 
The History of the Decline and Fall of the Roman Empire Vol. 3 [EPUB ebook] 

Gibbon offers an explanation for why the Roman Empire fell, a task made difficult by a lack of comprehensive written sources, though he was not the only historian to tackle the subject. Most of his ideas are directly taken from what few relevant records were available: those of the Roman moralists of the 4th and 5th centuries.