It has been a while now since people used the word "Augustan" as a term of approbation. The once common habit of looking back to antiquity for role models has ended up almost as dead as the Roman empire itself. Certainly, with the possible exception of spokesmen for Hizb ut-Tahrir, there is no longer anyone who would argue for ancient-style imperialism as the answer to our contemporary ills. Yet for all the appalled stupefaction with which most people nowadays tend to regard, say, an Alexander or a Caesar, there is one figure from antiquity who still remains the very archetype of the wise and noble ruler. When Russell Crowe, strutting his stuff in Gladiator, announced to the massed crowds of the Colosseum that he would have his vengeance, he did so, in his own words, as "loyal servant to the true emperor, Marcus Aurelius". No other Caesar would have done. Even in the multiplex, it seems, history's most celebrated philosopher-ruler can be a name to conjure with.

Frank McLynn, the author of a new biography of Marcus, shares in the near-universal admiration for his subject. "Unquestionably," he states flatly, Marcus "was the greatest of Roman emperors." This is not, he goes on to acknowledge, a judgment on his hero's political achievements, which mostly proved to have been built on sand; indeed, one of the reasons why Marcus's reign was always remembered so fondly by subsequent generations was precisely that it had marked the very last glimmering of Rome's golden age. Rather, McLynn's claims for his subject's greatness depend on his moral character - for here is an autocrat whose private notebooks, the so-called Meditations, were compared by John Stuart Mill to nothing less than the Sermon on the Mount. McLynn does not go quite that far, but he does certainly see in Marcus a reproof to Lord Acton's most celebrated dictum: here was a man who "had absolute power, but never used it for selfish, evil, despotic or corrupt purposes". Which is not to say that we are given no sense at all of Marcus's faults. Priggishness and an undoubted ruthlessness when it came to the defence of his own dynastic interests are set in the balance against his prodigious sense of duty and his customary mercy. The result is that rare thing: a portrait of a figure from ancient history that manages to be both psychologically nuanced and true to the original sources.

This is a study of Marcus's life and times, however, as well as of his character - a book that aspires to provide a portrait of the entire Roman world in the 2nd century AD. Like Marcus, who was never less than dogged in pursuit of what he felt needed to be done, McLynn is certainly assiduous in sticking to his objectives. Yet there are times when it can feel the most tremendous slog; exhausting as well as exhaustive. As the subtitle suggests, Marcus was a man of many parts - and there is not a part of him that is left unexamined here. Every campaign, every diplomatic initiative, every legal, constitutional and dynastic twist - each is minutely dissected. Indeed, by the end of the book, one is almost left sharing in the impatience of Commodus, Marcus's sociopathic heir, that his father took so long to die.

Among the most entertaining passages in McLynn's book are those devoted to a gloriously scabrous character assassination of the famous orator, and one-time tutor to Marcus, Cornelius Fronto. Drawing on the correspondence between the two men, which came to light as recently as 1815, McLynn portrays Fronto as compulsively orotund and prolix, a showoff who is never content with a single sentence when a whole battery of them will do. Here, perhaps, was a warning from history that McLynn would have done well to heed. Had he done so, then his book would surely have been a good deal lighter. Rather than listing every single province that Hadrian visited, for instance, he might have contented himself with saying that the emperor was well travelled; instead of likening Marcus to Nathaniel Hawthorne, or Cicero to Georges Simenon, he might have reined back on far-fetched and unhelpful comparisons; rather than discoursing windily on the career of Rousseau or Smuts, he might profitably have cut his entire last chapter. "We have come a long way from Marcus Aurelius," he comments at one point - to which the reader can only wearily assent.

In short, it is hard to know at whom this book is aimed. The general reader, whose interest may have been piqued by a reading of the Meditations or a viewing of Gladiator, is likely to find much of it intimidatingly dry, for when discussing the social and economic background to Marcus's reign, McLynn is much given to the lengthy rehearsal of lists. Conversely, specialists may well find many of its conclusions decidedly eccentric; McLynn is nothing if not aggressively opinionated. Yet with just a bit of cutting, and just a little less self-indulgence, this would be a most enjoyable and valuable book. "Remember," Marcus wrote, "that there is a proper dignity and proportion to be observed in the performance of every act in life." Not the least reason for reading about Marcus Aurelius today is that his meditations can indeed still offer all of us, historians included, worthwhile advice.

 Tom Holland's Millennium: The End of the World and the Forging of Christendom is published by Little, Brown