Eva Gabrielsson's sister returned to Sweden in the mid-1990s after eight years working in London. "She did not recognise the politics, mentality or how society worked," reports Gabrielsson, the architectural historian who for more than 30 years lived with Stieg Larsson. A campaigning journalist, anti-fascist investigator and finally novelist, Larsson became – after his sudden death aged 50 in November 2004 – the most-read and best-loved crime writer in Europe.

"Stieg and I had long talks with her about this," Gabrielsson recalls. To them, and to many other Swedes, a nation that for so long prized its civic ideals of compassion and community had somehow lost its soul. "The developments in the 1990s revealed greed, bonus systems, golden parachutes and corruption beyond imagination," Gabrielsson told me last week. "All in all, a total disrespect for the traditional Swedish values of honesty, equality and the common good. This was shocking, especially since nobody turned out to be responsible or held accountable... You could say that the veil of naivety about a dream-castle country fell with a bang, not with a whimper."

Intricately plotted, lavishly detailed but written with a breakneck pace and verve, the political thrillers in Larsson's Millennium trilogy have sold more than 15 million copies around the world. Five years after the death of this driven, heavy-smoking workaholic, who edited an investigative magazine, Expo, and also contributed for three decades to the anti-fascist journal Searchlight, the late whistle-blower still breaks news.

Niels Arden Oplev's film of his first volume, The Girl With The Dragon Tattoo, has yet to open here but has already enthused audiences across Europe. And a dispute over Larsson's estate still sidetracks attention from his books. He died with no valid will, his father and brother became his legatees by default, and a vigorous campaign in Sweden aims to secure Gabrielsson's rights.

MacLehose Press has just issued the English translation of the third volume: The Girl Who Kicked The Hornets' Nest (translated by Reg Keeland). Larsson's Lisbeth Salander, the sullen, wild and brilliant computer hacker and feminist avenger whose creation tore up the rule-book for heroines in crime fiction, comes face-to-face with her persecutors deep in the secret agencies of the Swedish state. Mikhail Blomkvist, the editor whose corruption-busting magazine Millennium has more than a hint of Larsson's Expo, again has to ally his skills with the elfin, pierced and near-autistic force of mind and nature that is Salander.

At length, the unfathomable Salander turns up in court to confront the establishment conspirators who made her young life a misery. She has a headful of "uneven stubble", 10 ear-piercings, a black leather miniskirt and a skimpy top bearing the legend "I am annoyed". You can say that again.

Beyond the books' flair for speed, surprise and sensation, Larsson – and Salander herself – have channelled an underground stream of fury and hurt among readers, in Sweden and far beyond. It seems to have much do with grief over a shared loss of innocence, and the withering of belief in a fair society. Individually, Salander emerges as a victim of multiple abuses, betrayed by psychiatry and social 'care'. Collectively, Sweden comes across as a nation undermined from the top by a cabal of thugs and spies whose responsibility for her ordeal gradually unfolds.

For Eva Gabrielsson, the novels' readers "recognise their own anger and frustration about everyday injustices and corruptions. This is also a frustration with the lameness of politicians." The books "clearly state that individual people do matter and may not be abused, lied to, misled or deceived for money, power or anyone's prestige."

Any reader of Larsson will pick up his disgust at male violence and at the official indifference that allows misogynistic cruelty free rein. Blomkvist has a kind of mission-statement: "This story is not primarily about spies and secret government agencies; it's about violence against women, and the men who enable it." Typically, Larsson interrupts the galloping plot to show how a good woman cop spontaneously takes revenge on an unprosecuted wife-beater when "something snapped in her". With Salander active and dangerous, we get to hear that snap a lot.

Of her late partner's preoccupation with gender-based abuse, Gabrielsson says: "In his mind it was a question which was constantly ignored, everywhere, and a systematic flaw in all of our societies." Yet there's nothing abstract about Larsson's rage against the degradation that those in power – in relationships, families, workplaces or states – feel entitled to inflict. In Hornets' Nest, we grasp every ghastly detail of Salander's teenage torment at the hands of a crooked psychiatrist with phoney diagnoses to defend.

Gabrielsson recalls that "Stieg had a deep mistrust of social workers and psychologists, given their power over people in times of need and weakness". She also remembers that "Stieg was especially furious with a certain small group of expert witnesses in Swedish paedophile cases, who for some reason too often landed on the side of the bad guys, calling the abused girl or boy disturbed and not trustworthy".

Yet this righteous agenda alone would never have topped the bestseller lists in a dozen countries. Their translator into English is the prolific New Mexico-based Steven Murray (aka 'Reg Keeland'). He reports that: "This was by far the most engrossing and most fun translation task I've ever faced... I could tell as I worked on them that they would be hits, but no one could imagine how big." So what accounts for these novels' phenomenal popularity?

"What strikes me most about Stieg Larsson is the way he kept his prose moving, even when in the midst of arcane digressions on any topic under the Swedish sun," Murray explains. "Part of it is creating characters that seem like real people, with all their talents, contradictions, and faults."

Then, of course, there's Salander. This tiny, mumbling, staggeringly gifted scourge of patriarchy can all but vanish from the action and still steer the thoughts of friends, foes and readers.

But did she spring fully armed from Larsson's imagination? "Salander is a fictional character, and is not walking the streets of Stockholm," Gabrielsson warns.

She does mention that "Salander's odd personality was made up from traits in individual people who Stieg and I associated closely with in Stockholm. It takes a lot of personality to inspire someone to write almost 2,000 pages, so needless to say, the models were all peculiar and active in society in some very definite, visible ways."

In Hornets' Nest, a tantalising double finale – first idyllic, then frenetic – reminds us with a pang that Larsson's punk princess cannot return. Sections of a fourth book were left on his laptop, but Gabrielsson has denied that a sequel might emerge.

So, like some legendary woman warrior, Salander disappears into her twilight of the tattooed, hi-tech gods. The novels as a whole mix this near-mythical dimension with a hothouse domestic atmosphere among tight-knit cliques. Larsson has made the literary moods of saga and soap opera converge – with suspense as the adhesive. And, behind the quickfire action, those great chords of moral and political witness continue to resonate. "Long after the entertainment factor has faded, I predict that readers will remember some of Stieg's serious concerns," says Steven Murray. "And if that isn't a mark of real literature, I don't know what is."