The figure of the Roman emperor still conjures up an array of familiar images, monuments and (often sordid) tales. These bygone rulers feature in our films; their portraits line our museums; and stories of their wars, dinners, sex lives and brutality continue to stir our imaginations and fuel our fears of unbounded power. It is this figure — not of any particular emperor, but of the emperor as a position or even an idea — that forms the basis of “Emperor of Rome: Ruling the Ancient Roman World,” the latest book by Mary Beard, whose previous works include the wildly popular Roman history book “SPQR.” Throughout, it is clear that Beard — a decorated retired Cambridge professor (and blogger and TV presenter) who excels at making the ancient world accessible to nonspecialist audiences — is herself deeply intrigued by the Roman emperor.