Edward Gibbon's The Decline and Fall of the Roman Empire -- It's a major work of the Enlightenment, a book that shaped how we moderns write history (and, for that matter, how we aspire to write in the English language), and it's now available as a free podcast thanks to Librivox. Or at least Volumes 1, 2, 3, and 4 are. (Click on each link to download the full zip files, which include many hours of audio. And please note that the remaining volumes are forthcoming.)
Published first in 1776, just as the US declared its independence from England, Gibbon's Decline and Fall looked to offer an empirical explanation for why Ancient Rome fell as a power, and he generally pointed to a decline in civic virtue among its citizenry (why bother fighting the Empire's wars when you can get mercenaries to do it?) and to the rise of Christianity (why worry about Rome when a better life, an eternal afterlife, awaits you?).
In part, Gibbon's work has endured because it speaks to questions that modern powers have on their minds. What brings Empires down, and what (implicitly) allows them to endure? These questions have a certain amount of relevance these days in an anxious US. And indeed Gibbon's name was immediately invoked in a podcast that asked whether America, today's empire, is on the brink. (Click to listen.) The parallels between Gibbon's Rome and the contemporary United States have also been directly explored by the prolific, young Harvard historian, Niall Ferguson. You may want to check out his October 2006 piece in Vanity Fair, Empire Falls. And depending on what you think, you can give time to his two books on Empire -- the first (and better) one focuses on the British Empire, and a second one devotes itself to the US.