On the Spartacus Road, By Peter Stothard

Ancient Rome was, without a trace of shame, a slave society. Captives from her innumerable wars were auctioned as chattels, and were seen as no more than "talking instruments". By the first century BC, slaves may have accounted for one third of the population of Italy, some two million souls.

The principle of servitude was unchallenged in the classical world. Even Christians said that slaves should obey their masters. Some thinkers encouraged owners to behave with decency and respect, but if one slave murdered his master all the others were put to death. In a criminal investigation, no slave could be held to have told the truth unless he had been tortured.

Slaves were feared as a potential fifth column. Mass revolts were an abiding nightmare, although few and far between. This must have been, at least in part, because a slave who worked hard and made himself agreeable stood a good chance of being freed, in which case his son could claim the prize of Roman citizenship. For that, it was worth putting up with a lot.

Agricultural labourers in remote haciendas had few opportunities for manumission. When the Thracian gladiator Spartacus escaped from his training school in Capua in 73BC, they flocked to his standard. He soon led an army of at least 70,000 men and proved an able commander, although eventually he was defeated and killed.

The particulars of his career are largely missing. Ancient historians treated his nine victories over Rome's invincible legions with embarrassed dispatch. This sketchiness has allowed novelists, composers, choreographers and film-makers to mould him into a freedom-fighter for their own times.

Peter Stothard decided to track Spartacus's marches across Italy, and On the Spartacus Road is the diary of his travels. Few traces of his hero remain. So, faute de mieux, the book is bulked out by digressions.

In fact, taken as a whole, it is a single huge digression, a circumvallation built around the most terrible experience in the author's life. About ten years ago, he contracted pancreatic cancer, among the most dangerous of malignant growths, which slips with promiscuous ease throughout the body. Death is the common outcome. With a scholarly eye for the curious detail, Stothard lets the reader know that pancreas means "'all-flesh' in Greek, a delicacy much prized by cannibals".

He endured both the disease and treatment, a painful alternation of chemotherapy and surgery. Displaying the sangfroid of the emperor's Stoic opponents, he nicknamed the invader "Nero". And, marvellously, he survived.

The passages in which he touches on the misery of the cancer patient are very moving – and the man himself steps out of the page, entire and present. But for much of the time, Stothard is invisible. Quite often his part in his own journey is obscure. So he refers to one of the Roman forum's best-kept secrets – a network of subterranean tunnels where gladiators probably waited. But did the author actually climb down and see them for himself, or take the information from a guide-book? We cannot tell.

This studied impersonality casts a slight chill over the narrative and in place of a "quest" we have, in effect, a collection of essays, loosely linked to various Italian places. We learn of all kinds of interesting people, most with little connection to the slave leader – Domitian's flashy, forgotten poet Statius, the amiable, able and self-important Pliny (junior), the water expert Frontinus, Hadrian's bohemian friend Florus, and so forth.

But Spartacus remains a shadowy figure, albeit that once in a while we receive an unforgettable insight into his impact on ordinary people - as when Stothard visits (or we assume he does) the ruins of a farmhouse in southern Italy probably destroyed by Spartacus's men. Archaeologists found some worn coins in what was left of an outdoor latrine, and the author nicely surmises that the farmer was carrying them in his tunic or the fold of his toga, as one did, and dropped them when surprised in mid-evacuation. For an empathising moment, the past becomes now.

Anthony Everitt's life of the Emperor Hadrian is published by Random House US