The Junior Officers' Reading Club, By Patrick Hennessey

Confessions of a laptop warrior

A tech-savvy Oxford graduate of the iPod generation spends the day spaced out on a high-decibel, high-adrenalin activity that leaves him feeling that "nowhere else sells bliss like this". As for the chilled come-down after those hours of brain-tingling rapture, it resembles "the end of some massive night in a hardcore warehouse". What has this ultra-cool dude, who drops phrases such as "post-modern irony" as readily as he customises Amy Winehouse lyrics, been doing?

He has been commanding a platoon of the Grenadier Guards as they turn the firepower of the British army (Nato approvals and UN resolutions all present and correct) on Taliban ambushers in the parched badlands of Helmand province, in southern Afghanistan, in May 2007. Mullah Omar's driver, it turns out, would not come home from that rave in one piece. "Something", this officer decides in the downtime after a near-orgasmic fire-fight, "was needed to shake us out of the dangerous enjoyment we were getting from it all."

Dangerous enjoyment, indeed. Connoisseurs of the finest front-line testimony composed after both world wars – or, later, Vietnam – know that (contrary to cliché) the classics of this genre seldom deliver a simple plea for peace. Instead, they tell the soldier's truth, which arises from its own occult place of passionate small-group loyalties, pitch-black eve-of-execution humour, ecstatic camaraderie, paralysing grief, and bristling suspicion of outsiders – all framed by a studied neutrality about the rights and wrongs of the carnage.

Patrick Hennessey – one grandfather a famous D-Day officer, but another a pacifist Welsh academic - studied English at Balliol. He signed up for Sandhurst ("Hogwarts with guns") in 2001 in part to spite the trainee lawyers and be one of "the guys in the room with something to say". After a ceremonial interlude palace-hopping in London – all booze, bearskins and "Texan cheerleaders" invited upstairs – the Guards lieutenant (later the youngest captain in the British army) found himself fighting the messy, modern "three-block war in three years, idle Balkan peace-keeping to tense Iraqi counter-insurgency to bloody Afghan combat". Having left the army, he is now training to become one of those lawyers himself.

From this total-immersion course in post-9/11 conflict, Hennessey has fashioned what must rank as the most accomplished work of military witness to emerge from British war-fighting since 1945. In salvo after salvo of nerve-shreddingly intense reports and reflections, built around the emails sent from camp as an "important therapeutic outlet", he lifts the "invisible curtain" that severs combat-bonded soldiers not only from civilians but behind-the-lines comrades as well. But The Junior Officers' Reading Club (his mates, by the way, devour far more DVDs than novels) will probably not change a single mind.

If you seek thin-brown-line heroics, it has gory gung-ho adventures to spare. Isolated British units in upper Helmand, along with their intrepid but untamable Afghan allies, throw themselves recklessly into one antediluvian close-quarters scrap after another: "surely this sort of thing went out with our grandfathers". But if you deem the "coalition" intervention in Iraq a neo-colonial crime and in Afghanistan (to cite Talleyrand) worse than a crime, a blunder – then this best and brightest of the occupiers will tell you that blasting "Terry" Taliban with maximum force out of the dusty compounds he shared with non-combatants was a gas: "just how easy it all was, how natural it all felt and how much fun". The fun abruptly stops when close friends begin to lose limbs to shells a few feet, then a few inches, away. Hennessey's unit suffered a one-in-three casualty rate.

His Grenadiers have a reputation as "unreconstructed traditionalists"; they certainly detest the RAF more than the Taliban. That faith in the old ways shades his narrative as well. Emotionally, this is battlefield reportage in a classical vein, with every well-marked step on the soldier-writer's march taken in its swift stride. Sandhurst means the soft cadets' bewilderment yielding to grudging respect for grizzled NCOs ("Don't salute me, sir, I fucking work for a living"). Iraq entails thumb-twiddling in camp and "history happening" amid the wreckage of Baghdad. In Helmand comes not just "the rapture of a real fight" but age-old scorn for meddling hangers-on and "presumptuous journalism", the "us against the world" bonding of a unit under fire and, in the frozen aftermath of trauma, an "endless debauch" that fails to ease the pain.

So far, so canonical. Yet his tale's true originality lies in Hennessey's media-saturated reflections on media-shaped campaigns. Here, postmodernism dons desert fatigues. These kids act like a band of brothers in part because they watch Band of Brothers. Prior to one encounter, they prime themselves with the supremely silly Spartans-vs-Persians anime, 300. As for their video montages of real skirmishes, "the work of a few hours" on standard laptop software, they have "changed the way soldiers went to war". When it comes to the soundtrack to Basra, forget Vietnam-era rock: "the rampant consumerism of gangsta rap struck a chord with our little oil war". No surprise that hard-core action in Helmand first "felt more like being on set than real life".

Hennessey draws deeply on the close-focus, high-octane buzz of front-line "new journalism" (above all, Michael Herr's Vietnam masterpiece Despatches). He puts the gonzo rush of Hunter S Thompson's delirious prose to good, if ultimately tiring, use. Yet his subtler, introspective moments linger longer.

Under siege in Sangin, he reads Alan Hollinghurst's The Line of Beauty, which triggers nostalgic pangs for the parties of peace. Longing for home, he still knows that, if he gets there, home will bring an ache to be back under fire, with "all the things we learned about ourselves". And among those things is the yearning for life before this soul-shaking intimacy with combat. Hennessey has now quit his disputed wars. He may shed a uniform but – surely - he cannot abandon the rare gift revealed in this extraordinary book.