I, Claudius (1934) by Robert Graves. It's a great piece of historical fiction, based in large part on Graves' translation of The Twelve Caesars by Suetonius. The book feels remarkably modern and personal, though; it’s the secret autobiography of Claudius, an historian in ancient Rome. When I first read it, I believed that Claudius had really written it!

Beyond that, I, Claudius is becoming more and more relevant to contemporary American politics every day. It's really astonishing.