It's the fastest-selling novel for adults of all time – and it's very adult in content. Why have millions of women been seduced by Fifty Shades of Grey, asks Zoe Williams

It's pointless to deny that there's something going on here: EL James has now sold 4 million copies of her Fifty Shades of Grey trilogy via her UK publisher, Random House, to add to the 15 million (it beggars belief) that have been shifted in the US and Canada. In three months. In the UK, it's the fastest-selling book ever in both physical and ebook incarnations. There's just been an extra print run for the UK market, to meet demand: 2.75 million copies. It's the fastest selling adult novel of all time. By which they mean "it's the fastest-selling novel of all time that isn't Harry Potter". But its content is, of course, rather adult.

The trilogy features Anastasia Steele, who falls in love with Christian Grey, a troubled young billionaire who likes sex only if he can accompany it with quite formal, stylised corporal punishment. The narrative drivers are pretty slack – improbable dialogue ("I'm a very wealthy man, Miss Steele, and I have expensive and absorbing hobbies"); lame characterisation; irritating tics (a constant war between Steele's "subconscious", which is always fainting or putting on half-moon glasses, and her "inner goddess", who is forever pouting and stamping); and an internal monologue that goes like this … "Holy hell, he's hot!"; "No man has ever affected me the way Christian Grey has, and I cannot fathom why. Is it his looks? His civility? Wealth? Power?" Yuh huh. Civility puts me in a blue funk too.

In normal circumstances, it would be lazy, but here, it is more like a shorthand. James writes as though she's late for a meeting with a sex scene. Here, her voice is quite different: meticulous, inventive, radical and conflicted; Grey is only interested in a dominant/submissive relationship (with these "hard limits" – no fire, no faeces, no blood loss, no gynaecological instruments, no children or animals, no permanent disfigurement, no breath control and no direct electricity – I paraphrase for brevity). Steele just wants a regular boyfriend (or does she? Yik yak yik yak). This is Fifty Shades of Grey I'm talking about. We'll come to Fifty Shades Darker later. Goddammit. I've been infected by James's ominous, staccato delivery. After 1,600 pages of the stuff, you will too. I'm doing it again. I can't help it.

There is a little light spanking in Jilly Cooper (Octavia, Rivals), and the romance genre (as distinct from chicklit) would be many pages lighter if nobody ever got tied to a bed with a scarf, but this is in a different league. Its popularity has come as a bit of a surprise to publishers, who thought they knew what women wanted. It must be a bit like being married to someone for 20 years, and suddenly finding out they like fisting. People who like to trace all new trends back to new technology have offered this explanation – that women who wouldn't be seen dead reading smut on the tube could read it on their Kindle, and this launched a whole world of sales.

The unexpected element is that the shame of erotic fiction is largely in the imagination, and once people had read it, they felt happy to discuss it openly. It was word of mouth that launched the paperback version on the back of the ebook.

Where do you stand on erotica in public spaces? Someone in a tube carriage last week with three people reading the paperback (and God knows how many reading it on their Kindles) tweeted, "isn't it a bit early for that sort of thing?" – as though there were an erotica yardarm, and we all knew when it was. After lunch? When the sun goes down? It seemed a bit random, yet I can see why he'd query the wisdom of summoning a sustained erotic vignette on one's way into work. But what do I know? I work at home. Maybe people do that all the time.

Consider, furthermore, the way high culture and low culture have collided. It's long been acceptable to read the Financial Times and also watch the Eurovision Song contest, read Philip Roth as well as Marian Keyes. Because erotica is niche to start with, this revolution took longer to reach it, and only now have we loosened up a bit. By this reckoning, Fifty Shades is just Mills & Boon for the generation that would once have been embarrassed to be seen reading Mills & Boon.

No, there is more to it than that. First, the reason sex scenes are so difficult to write is the gear change, rather than the sex itself. It is extremely difficult to write a regular story spliced with sex, just as it would be difficult to tell a story interspersed with explicit sexual detail. That's why the Bad Sex Award exists, and is so easy to bestow. In the very act of describing sex as an incidental, you create an excruciating sex scene.

EL James. Photograph: Michael Lionstar

James's sex scenes are not incidental, they are the meat of the plot, the crux of the conflict, the key to at least one of and possibly both the central characters. It is a sex book. It is not a book with sex in it. The French author Catherine Millet wrote: "For me, a pornographic book is functional, written to help you to get excited. If you want to speak about sex in a novel or any "ambitious" writing, today, in the 21st century, you must be explicit. You cannot be metaphorical any longer." I'm not sure James's writing is that ambitious, but she has certainly understood the bit about not being metaphorical.

As history is written by the victors, so S&M is written by the Ss, and the problem with sadists is that they exaggerate. They're not looking at it from the masochist's point of view – it's in their job description not to. If the Marquis de Sade thinks any garden– variety submissive is going to get a kick out of having their back broken on a cartwheel, he's dreaming. Conversely, two opposite predilections, across a very broad scope, might easily collide in a fantasy written from the perspective of the masochist or naïf. So that's the popularity of volume one.

The second volume is a bald and rushed go at monetising the brand. The deviant stuff is largely excised, and the move towards mainstream sexual endeavour seems to bore the author. Her fantasies turn instead to what presents she'd like if she fetched up with a billionaire (an iPad. An Audi. No, a Saab! Nope, I feel cheap. OK, OK, just the Saab, and some clothes, ooh, a bikini, for $541 … what a terrible waste, and yet how pert my breasts look).

Now we're looking at a book you'd be embarrassed to be caught reading on the tube. Small habits begin to grate: the way everybody always seethes, scolds, smirks or whispers and nobody ever just says; the way his eyes are constantly blazing, and she is constantly biting her lip.

The link between volumes is so clumsy that you have to look away ("He thinks he doesn't deserve to be loved. Why does he feel that way? Does it have to do with his upbringing? His birth mom, the crack whore?"). The need for a plot invites in some true gothic horror show and, stripped of his deviations, Christian Grey is just a controlling, unpleasant man whom, even 30 years ago, no sane heroine would ever have married, however Holy-hell-shit-I-can't-breathe hot he was.

The third in the series, Fifty Shades Freed, is … Oh what am I doing? You're going to read it. Of course you're going to read it. You've probably already read it.