Robert Harris was an influential journalist and BBC reporter before he turned to writing fiction. His close friends have included Peter Mandelson and Roy Jenkins, and most of his ancient Roman politicians resemble figures in contemporary British politics. He has a pitch-perfect ear for class snobbery, hypocrisy, parliamentary posturing and bluster. His best episodes bring crucial behind-the-scenes moments in Roman political skulduggery to colourful life. He writes with swaggering confidence.

Dictator is his ninth historical novel. It is also the third novel in a trilogy set in the mid-first century BC, centring on the Roman orator Cicero. The trilogy is narrated by Cicero’s slave and secretary, Tiro, the inventor of our modern system of shorthand, whom Cicero rewarded with emancipation.

Imperium, the first novel in the trilogy, began with Cicero launching his career as a public figure by prosecuting the corrupt governor of Sicily; in the second, Lustrum, Cicero became consul and was hounded into exile by the sneering, arrogant aristocrat Publius Clodius Pulcher. In this final instalment, which begins in 58BC, Cicero returns from exile. It ends shortly after his brutal execution in 43BC, when his name was put on the hitlist comprising everybody who opposed the authority of the ruthless Second Triumvirate (Antony, Octavian and Lepidus).

Dictator is climactic in every sense: loyal readers have now been waiting for nearly a decade to discover how Harris would deal with the most famous moment in Roman history, when the Republic staggered to its gory finale with the assassination of Julius Caesar. Harris does not disappoint. His Caesar is a menacing, genocidal psychopath, but so charismatic that everyone in Rome, including Brutus and the other assassins, is left strangely bereft in the days of eerie crisis following the Ides of March in 44BC.

There are, however, profound weaknesses in Harris’s writing. He fails to exploit the possibilities inherent in using Tiro as a narrator: even the best-treated slave would surely have occasionally become irritated with such a demanding, inconsistent and egotistical owner as Cicero. The political analysis is often crude, as when Harris makes Cicero ask ponderous questions straight off an undergraduate ancient history exam (“Can a constitution devised centuries ago to replace a monarchy, and based on a citizens’ militia, possibly hope to run an empire whose scope is beyond anything ever dreamed by its framers?”). Harris has superficially absorbed a commendable range of ancient sources, from Cicero’s own huge oeuvre (letters, speeches, philosophical treatises) to Caesar’s Commentaries and Plutarch’s Lives. But for a political journalist, he adopts the agendas of the ancient writers with startling naivety. He reproduces, without a trace of cynicism, the most salacious ancient propaganda – for example, that Caesar was addicted to depilatory treatments.

The uncritical reading of ancient authors is clearest in his characterisation of famous females. Terentia, Cicero’s wife, is the shrewish snob ancient men said she was; Clodia is a treacherous vamp. Did Harris ever stop to ask whether Cleopatra’s alleged hyper-sexuality was an unpleasant product of Augustus’s propaganda machine, before he made Tiro recall that she “had huge charcoal-black eyes and a painted ruby mouth – an aged slattern’s mask even at the age of eleven”? He is on record as liking to write “at a gallop”, and as having an aversion to any form of revision or refinement of the first draft of his novels. This often shows. A bit of revision might have improved his trite description of some senators “having forsaken the sun of Campania to bask in the warmest sun of all: power”. He could be awarded a prize for the most cliched literary account of a death, when Cicero’s beloved daughter Tullia expires: “a long sigh, accompanied by a slight tremor along the length of her body, and then a profound stillness as she passed into eternity”.

Despite its manifold faults, I enjoyed Dictator enormously. Harris loves Cicero – like the author, he was a self-made man from a relatively humble background – and communicates his own fascination with the epic showdown that constituted the fall of the Roman Republic. Dictator may be a sensational political thriller rather than serious documentary fiction, but it is often funny and touching. I could not put it down.

Edith Hall’s Introducing the Ancient Greeks is published by Bodley Head. To order Dictator for £16 go to bookshop.theguardian.com or call 0330 333 6846. Free UK p&p over £10, online orders only. Phone orders min p&p of £1.99.