Lena Dunham's first book, like Dunham herself, has accrued a cultural significance that is much greater than the sum of the parts. The daughter of two New York artists, she has been the focus of US media attention since she was a child, to a degree that most people would find bewildering – though she, according to her memoir, took a dissociative pleasure in it. When she was 11, she was interviewed by US Vogue about her thoughts on fashion: "I find Calvin Klein really hard to respect because he's everywhere. I view him as a clothesmonger," the 11-year-old mused. When she was 16, the New York Times, for no obvious reason, covered a "vegan feast" she threw for her friends where shoes were banned: "I just thought it was sort of bohemian-seeming," the teenager explained.

Ever since it was announced that, on the back of her debut film, Tiny Furniture, she would be directing, writing and starring in the HBO series Girls, Dunham has been dubbed by the media as the "voice of her generation" – "or at least, a voice," her character Hannah amusingly slurs to her parents while intoxicated in the pilot episode of Girls. Dunham's fans and detractors have, ever since, lionised and demonised the twentysomething to the point that she now represents whatever the hell you want: the blogging generation! Feminism! Misogyny! Just stick a reference to Dunham into your zeitgeisty feature and the youth-obsessed media will clamour to publish it.

At the other end of the spectrum, blogs such as Gawker and Jezebel (Gawker's nominally feminist offshoot), staffed largely by people Dunham's age, can't kick her enough. The biggest complaint about her is that she represents all that is wrong with an over-privileged, nepotistic, Caucasian-focused slice of America. When it transpired that there were almost no people of colour in the first series of the show, critics cried racism in a way that no one ever did about the similarly New York-based and generation-defining Seinfeld, Friends or Sex and the City. Dunham has said that this isn't an excuse (she is good at taking criticism, far better than most people who are as much in the public eye as she is), and she's right. But the relevance of that comparison is twofold: first, it's a reminder that, in fact, New York is much more segregated than a lot of people appreciate. I grew up in a similar neighbourhood to Dunham and went to a similar school, and I didn't know a single black person who wasn't my babysitter until I moved to London. What's more, the expectations placed on Dunham to represent all life experiences of everyone of her generation were not only absurd but proved how good a writer she is – if she was bad, no one would have cared what she thought (although none of her critics would ever admit that). Dunham was never going to present an everyman view of the world (whatever that means).

But anyone who could look beyond the general media nonsense surrounding her found a precociously smart and supple writer, which should not be a surprise. After all, the New York Times publishes vacuous trend pieces about random people every week, but HBO does not hand out TV series to any old random Calvin Klein-hating, self-consciously bohemian twentysomething. Tiny Furniture has a raw honesty to it that makes Karl Ove Knausgaard look like Whit Stillman. The first two series of Girls (which recently finished its somewhat weaker and broader third series) might have presented a privileged world, but the excellent dialogue and emotional precision outweighed all. At the end of one episode, Adam (Adam Driver), who had heretofore been portrayed purely as Hannah's unknowable lust object, turns the dynamic around and yells at her for being so self-obsessed: "You never ask me anything besides 'Does this feel OK?', or 'Do you like my skirt?', 'How much is your rent?'. You don't want to know me. You want to come over in the night and have me fuck the dog shit out of you and then leave and write about it in your diary!"

Which brings us to Not That Kind of Girl, a collection of autobiographical essays, for which Dunham was paid a much-reported $3.7m (£2.3m) by Random House, a figure that is simultaneously ridiculous and absolutely right. The reason she was paid this much is three-fold: first, and less relevantly to the publisher, she is a skilled writer, but there are lots of skilled writers in the world and they don't get seven-figure advances; second, she is Lena Dunham, publicity catnip with devoted fanbase; third, because it is a self-exposing memoir.

Where once misery memoirs were the vogue in the publishing world, now it is books by young women writing about what is usually described as "all their flaws", which means everything that happens in their vaginas, from masturbation to menstruation, from sex to cystitis. Clit lit, I guess. Caitlin Moran does this better than anyone else, and she makes it look easy, which it is not. Some will mock Dunham's book for its casual references to her privileged, beyond-parody hipster life: her father's therapist, her mother's nutritionist, the Puerto Rican man she dated "with a tattoo that said MOM in Comic Sans". But it seems a bit rich to demand that young women reveal all about their lives and then criticise them for living the wrong kind of life. Less justifiable, though, is the self-obsession.

Dunham writes that when she was younger the only thing she liked about sex was "it was maybe the only time I believed anyone besides me even existed". Judging from the excellent characters she created in Girls, Dunham has long since grown out of that and has empathy in spades, but it is not clear at all from her book. She says that Not That Kind of Girl was originally inspired by Helen Gurley Brown's Having It All, but a more instructive comparison would be with David Sedaris. Sedaris writes about himself, his family and his neuroses brilliantly, because he always looks outward in his books, describing the world through his own unique perspective. Dunham, by contrast, is an inward writer, and even though she is a pleasure to spend time with, reading this book feels a little like being squashed up inside her bellybutton. After all these pages, I still have no real idea what her family are like. At one point, I started counting the number of times Dunham wrote about vaginas in the book (hers, mainly). I quit when I got to 25. There's sexual honesty, and then there's just sticking your head up your vagina.

Some commentators have argued that clit lit it is helpful to teenage girls because it teaches them that bad sex is par for the course. But the other side of the coin is that this genre suggests the only truly interesting thing about a woman is her most intimate personal life. Certainly, Dunham sees her self-exposure as a benevolent feminist act: "There is nothing gutsier to me than a person announcing that their story is one that deserves to be told, especially if that person is a woman," she writes. But saying a story deserves to be told doesn't mean that it inherently does. Later, she writes that a teenage boy once told her that the fact she takes her clothes off so much on the show "made me feel better about myself". But she does not explain why, as the director, she cast conventionally beautiful actors to play all the other characters.

Dunham has now made what are essentially three versions of her autobiography by the age of 28: a film, three seasons of a TV show and a book, and the gruel is running thin. The anecdotes in the book often feel like rejected ideas for Girls episodes, which, in fact, at least one of them was. Dunham describes her need to share as a compulsion: "I have to tell my stories in order to stay sane," she writes. But she also needs an editor (or an Adam) who can say to her, "Great, but perhaps we don't actually need to publish a 10‑page chapter consisting purely of your food diary?" She is a brilliant talent who will write better books than this – and, really, who can blame her for taking the money and running? It's a shame, though, that her US publishers didn't take more time with her instead of rushing to cash in on the Lena Dunham industry. But then Dunham is probably used to that by now.

• To order Not That Kind of Girl for £12.49 (RRP £16.99) go to bookshop.theguardian.com or 0330 333 6846.