Half a lifetime ago, when I was living in Rome, I kept Suetonius’s Lives of the Caesars in the Penguin translation by Robert Graves as a bedside book. It’s a fascinating book, full of good stories, scandalous anecdotes, and intelligent observations. The question for the reader is what to believe. Gaius Suetonius Tranquillus, author of some dozen books, among them Lives of Famous Whores and Physical Defects of Man (neither of which, sadly, has survived), was born in about AD 69, the Year of the Four Emperors (Galba, Otho, Vitellius, Vespasian), and was at one time secretary to Hadrian (emperor from 117 to 138). He had access to the imperial libraries, but we don’t know which books he used as the source material for his work – the Caesars begins a hundred years before his own birth. So how reliable is he? The same question applies to the other, greater historian of the early empire – Tacitus, born AD 56 – and also to Dio Cassius (c AD 165–235).

Delighted but often puzzled by Suetonius, whose brief biographies tend to be assembled without much seeming grasp of structure or respect for chronology, I wrote what I considered a companion volume, published as The Caesars in 1983. Now Matthew Dennison has had the same idea, and has made a very