Charlotte Roche — a dainty, giggly brunette with the manners of a countess — does not look like the sort to commit the phrase “rectal goulash” to print.

She does not look like the kind of woman who could write a novel set entirely in a hospital proctology department. She does not seem capable of describing, in nauseating detail, the picaresque sexual adventures of Helen, her precocious, bacteria-loving 18-year-old heroine, who is laid low when an intimate shaving injury becomes infected. No, Roche, 30 — dressed in a demure floral dress and brown penny loafers — looks as if she should be filing index cards at a public library.

If Roche looks wrong for her new role as the author of Germany’s most shocking book — Feuchtgebiete, or Wetlands in English — that’s hardly surprising. She is not easily pigeonholed. For 12 years, she was a lightly famous television presenter on the music channel Viva: a Miquita Oliver or an Alexa Chung. She was born in High Wycombe, but her parents took her to Germany as a one-year-old, where she spoke English at home and German at school. Now she lives in Cologne with her husband, a television producer, and five year-old daughter. Yet her English has developed endearing faults: she says “bits and blobs”, mistakes “screamish” for “squeamish” and calls her retired father a “pensioneer”.

Her book caused a delicious outrage when it was published in Germany last year. Women fainted at public readings. Critics praised it as a feminist masterpiece or denounced it as a “masturbation pamphlet”. Either way, everyone read it — at the last count, almost 1.5m copies of Wetlands had been sold in Germany. And if they haven’t read it, they’ve seen it: a stage adaptation sold out for weeks in the respectable market town of Halle. In short, Roche is Germany’s most famous author. Imagine JK Rowling had written Hairy Trotter and you get the idea.

Roche never dreamt of writing a novel. She’s not even much of a bibliophile: in the five years since her daughter was born, she has read only one book, The Great Gatsby, and its influence on Wetlands is slim. In fact, the first words of fiction she ever wrote (“As far back as I can remember, I’ve had haemorrhoids”) became the first line of the novel — a novel sparked by a Damascene moment in a local pharmacy, where Roche saw the vast array of feminine hygiene products on display and decided to write a manifesto about “how stupid we women are about our own bodies”. Her only plan was a five-word checklist she kept on her desk, which read “smegma, shaving, spots, masturbation, menstruation”.

“At first, I wanted to write a pamphlet, about how everyone has become a hygiene maniac, then I thought, it’s too boring, and I invented Helen,” Roche says breathlessly. “I invented someone who is much cooler than I am, who is much more free and open-minded than I am, who could explore all the taboos. As I was writing it, I exaggerated all the time, and it got more and more disgusting and hilarious.” Disgusting we can all agree on. The episode concerning an incident of haemoglobin-rich cunnilingus reversed my breakfast.

“Well, that’s a big compliment to me,” Roche says, laughing. “It happens that people faint in my readings. They get so worked up in it — it’s either the sexual stuff or the stuff about menstruation. They are such taboos in people’s heads that, when I go there and keep going there, they can’t take it. I’m proud that I make people faint with words.”

So, Roche wanted to shock people? “No,” she says. “It was just meant to be an honest book about the female body. You know, people are surprised by this, but if I knew someone like Helen in real life, I would be extremely disgusted. I would never say, ‘Let it all hang out and let’s be natural about it.’ I clean myself, which surprises people. I shower every day. I shave all the parts you’re meant to shave as a woman nowadays.

“Obviously, nobody is like Helen. But the fun in writing the book was getting all the secret stuff out — all the things women are ashamed about. For instance, with my husband, I don’t leave dirty knickers lying around. I hide them, take them myself to the washing machine. Why am I so embarrassed, even in my own flat?

“The feminist angle to the book is this: I think women, now, have to have this clean, sexy, presentation side to their body. At any time, you must be available for sex, and you can just strip naked and look super. That’s a high pressure, and the joke in this book is saying, ‘Women shit, too, you know.’ I know there are men who will find that hard to accept, because they are thinking, ‘I want to f*** a clean woman.’ ”

Wetlands delivers a robust examination of the notion of the “clean woman”, but, as with most novels conceived politically, it fails in what should be its first objective: to make the reader care about its protagonist. Nowhere, for instance, is Helen described — an odd omission, given that I have an indelible image of what her haemorrhoids look like (cauliflower, naturally). Although Roche argues that this lack of essential framing is deliberate (“In my head, she looks like me”), I can’t help thinking it undermines her greater point — that women are more than the sum of their orifices.

There is, however, a subplot to Wetlands that, had it been elaborated, might have made for a more compelling narrative — Helen’s desire to see her divorced parents reunited. This theme comes from Roche’s own childhood, which was, she says, desperately unhappy. Her parents — a superliberal mother, who allowed Charlotte to have sex in the house from a young age, and her engineer father — divorced when she was five, “and not in a good way”. After the separation, her mother hopped from one husband to the next and from one town to the next. She had five children, and adopted one, and now works for an NGO in West Africa.

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