Anyone hunting for clues to the inspiration behind the bestselling novels of Robert Harris need not look far. Nestled amid the floor-to-ceiling shelves in his spacious, cigar-scented study at home in Berkshire – packed to the brim with histories and political biographies – is a large bust of the emperor Julius Caesar. On his nearby desk is a life-sized replica of the only extant sculpture of the Roman general Agrippa, titular hero of his latest historical blockbuster. Harris has spent many hours staring at the bust – “the face of someone you would not want to cross,” as he describes it – and pondering the motivations of the commoner who rose to glory 2,000 years ago as right-hand man to Caesar’s adoptive son Augustus.