Chimamanda Ngozi Adichie's third novel, Americanah is the story of two Nigerian émigrés who love and lose each other across continents and years. It is a book about leaving, and loneliness and the intersection between class and race, all of which makes it sound rather hard work – unjustly so. It is a book about hair: straight versus afro; and discreet tensions, not just between white Americans and Nigerian immigrants, but between Africans and African Americans, between the light- and dark-skinned, between new and established immigrants, and its frankness – in particular on the subject of gender – has upset some people. "I knew that was coming," says Adichie. "I can't write a book like that and then go, 'Oh my God, they're upset.' But my intention wasn't to upset." She smiles. "It's just that I'm willing to if that's what it takes to write the book."

We are in New York, which Adichie is visiting from her part-time home in Maryland, and where the following evening, Americanah (the word refers to Nigerians returning home after living in America) will win best novel at the National Book Critics Circle awards, beating the favourite, Donna Tartt's The Goldfinch. At 36, she has three much-acclaimed novels behind her – her first, Purple Hibiscus (2003) was longlisted for the Man Booker prize and shortlisted for the Orange, which she won in 2007 with her second novel Half of a Yellow Sun, and Americanah is just out in paperback – and a film version of Half of a Yellow Sun out next month. Adichie is in a position to do precisely as she pleases – such as divide her time between the US and Lagos, where she owns a home. In fact, if her husband didn't work in Baltimore, she says, she would probably "live in Lagos all of the time and just come to the US when I want to go shopping." She hoots with laughter.

Fashion is a big part of Americanah. Ifemelu, the heroine, comes to the US at 19 and is faced with a choice: in this new country, what version of herself will she be? She adopts and then sheds a fake American accent. She burns her scalp with hair relaxant to "Europeanise" her hair and then reverts to an afro. (The novel opens with a scene in a hair salon outside Princeton where she sits for six hours while the African hairdresser listens, incredulous, to her decision to go home.) She starts writing a blog called Raceteenth or Various Observations About American Blacks (Those Formerly Known as Negroes) by a Non‑American Black, cataloguing the judgments and sympathetic failures that this description entails, including the fact that in America, she observes, everyone is constantly being diagnosed with mental illnesses that don't exist on her continent. No one, Ifemelu muses, has a "panic attack" in Kinshasa. "Did things begin to exist only when they were named?"

These aspects of the novel describe Adichie's own experience of leaving Nigeria at 19 to attend college in the US – although Ifemelu stays away for 13 years without a trip home, while for Adichie it was "only" four. She is shrewd on the subject of white American self-confidence, the ease and largesse of deep-seated privilege. There is the husband of the woman she babysits for, "brimming with his awareness of his own charm". There is the man she dates, a white American called Curt who is achingly sensitive and clocks many of the racist slights she is subject to, meanwhile displaying "an infantile quality … that she found admirable and repulsive". When Ifemelu dates Blaine, an African-American professor at Yale, she herself is found wanting. "She was not sufficiently furious," writes Adichie, "because she was African, not African American." It is only when Ifemelu comes to America that it occurs to her to think about race at all. "I only became black when I came to America."

It is a peculiarly Nigerian response, Adichie says, one rooted in her country's history and in cultural attitudes, which, she laughs, have made Nigerians hated and resented across Africa. She grew up in a middle-class household, the daughter of academics and the fifth of six children. While her siblings all followed her parents' wishes and became "very responsible" – "the doctor, the pharmacist, the engineer" – Adichie asked to drop out of medical school and, if she passed the scholarship exam, go to America to study "communications". Her parents were baffled but gave in. "They thought I was slightly strange, but my parents are quite lovely; they're progressive. Still, it is a standing joke in the family that I say to them: 'The only reason you supported me is because my sister was already a doctor.' If not, I think they would have said, nobody leaves medical school, and especially not for reasons such as 'I'm not happy.'"

Compared to the strict educational environment in which Adichie grew up, the US education system seemed extremely slack. In Americanah, Ifemelu marvels at how students open their mouths without having anything much to say; how everyone gets an "A" and can take tests more than once. How they are "all flush with knowledge, not of the subject of the classes, but of how to be in the classes".

By comparison, Adichie says, she felt she had no appropriate schtick. "I remember my first year in undergrad I was sitting in class and just looking around, utterly confused. I thought, what are they saying? It was kind of a performance. And I felt so inadequate because I didn't know how. And I think at some point I learned to play the game, but it's just not me. It's very hard for me to bullshit."

Given the way men and women behave in meetings, one imagines her inhibition was a gender thing, too.

"Yes. When women do say something, not only is it more likely to be ignored, but the women themselves are accepting of it being ignored. So they're less likely to push back. That happens less often in Nigeria. Women in corporate settings are more likely to be vocal."

Why does she think that is? "Because although there's a lot of gender bullshit in Nigeria, I think women in the west have a lot more invested in being liked. And being liked if you're female means a certain thing. So in workplaces, women who are bosses in Nigeria are fierce. The people who work for them, men and women, respect them. But, these are women who very keenly perform gender stereotypes when they go back home. And if they give a public interview, they have to say, 'My husband supported me and allowed me to …'."

In 2012, Adichie made a speech about feminism at a TEDx conference in London, which, a year or so later, Beyoncé sampled on her single "Flawless", and she is at her best on this subject. The stick she got for Americanah was not, to her amusement, about race, but about gender, in particular about Ifemelu's unladylike character. "I've heard from many women, particularly black women, who really disliked Ifemelu."

Because she was too hard? "Yes. I hear that they said she wasn't grateful to have a good man. That is my point! Why does she have to be grateful to her man? Do we have the same standards for men? We don't. So she cheats on a good man for no reason. And she's crucified for it, but if we turned it around and she were male …" Shrugs. No big deal.

The most misread character in the novel is Kimberley, a woman Ifemelu babysits for and who some readers have erroneously called racist. Adichie says she wrote Kimberley with someone she knew in mind and intended the mockery to be affectionate. (For example: Kimberley is always pointing out plain black women in magazines and going on about how beautiful they are, in a hapless effort to show Ifemelu she isn't racist.)

"I don't think of Kimberley as racist. She's a type: a liberal, well-meaning American. So well-meaning she doesn't know how to deal with race. There's something about it that I find very funny. Talking about race in the US, it very easily becomes about demonising."

There is one stand-out racist incident in the novel, says Adichie, when a carpet-cleaner comes to the large house of the couple she babysits for and mistakes Ifemelu for the lady of the house. This happened to Adichie when she was babysitting in the wealthy suburbs outside Philadelphia.

"I remember opening the door and his face was such a cliche; he looked at me and his whole face fell. You know: I can't believe I'm working for a black person. And he had such an attitude. And then when I said, 'Mrs So and So said you were coming', he changed and suddenly was my friend."

It was fun to put all this stuff in the novel, she says, and to harvest her friends' experiences as immigrants to the US and Britain. Ifemelu's long-lost love, Obinze, goes to London and has a generally awful time, ultimately getting deported after a failed green card marriage attempt. She based this on the testimony of several friends who had been in that situation, which struck Adichie as the kind of thing – "the secret society of immigrants, where you meet people and immediately get a sense that they must have come undocumented" – that hasn't been much tackled in fiction. It also gave her the chance to write across a spectrum of idiomatic English, from London to Lagos to the east coast of America.

At this point in her life, she says, her own English is "completely confused. My sensibility is Nigerian. I went to school in the US. After Lagos I deeply love London. So I grew up spelling the British way. And then I went to the US and was suddenly spelling "colour" without the "u". My father is horrified by that. I was determined for so long not to use certain American expressions, because I came to the US with this Nigerian/British arrogance of believing English is as it is spoken in England. Americans would say certain things and I would think, people can't speak English."

When she is writing she is, of course, able to range far outside her own experience, something she did most strikingly in Half of a Yellow Sun, a story of the 1967-70 Biafran civil war in Nigeria, in which her father lost his own father and everything the family owned. Written when she was 29, it was, simultaneously, a historical novel and a deeply personal story, and she was terrified that Nigerians, generally, and her father in particular would object to it. "There were people who'd lived through the war who were writing 'What does she know about the war?' One man wrote to me and said, 'Your father wrote that book for you.'"

There were two people above all others who she wanted to read it: her father and Chinua Achebe. Her agent sent it to the latter without telling her, and then called her one day and told her to sit down, she had good news. Then she read Adichie his comment – "We do not usually associate wisdom with beginners, but here is a new writer endowed with the gift of ancient storytellers" – at which she burst into tears.

As for her father; she left the novel with him the day before she left Nigeria, so she wasn't in the house when he read it. A few days later, he sent her a text: "Call me, I've finished." Terrified, Adichie made the call.

"And then he said to me, 'I knew the novel would be good; I didn't know it would be this good.' And then he said thank you, 'Our story has been recorded.' I remember thinking, OK, it's over. I don't care what anybody else thinks. My father was central and he was so generous; I had used so many of his stories. It's still very painful for him. And then writing it in a novel where people also have sex and scandalous things happen." She laughs. "I was like, oh my God, I don't know what daddy's going to think!"

Adichie says she doesn't read her reviews, but bits filter through from friends and the thing that has pleased her most about the response to Americanah is how funny Americans think it is. "They had a sense of humour about it, which takes a certain grace."

There is nothing like emigrating to encourage a sense of condescension towards the motherland, and absence, she says, has made her both love and criticise Nigeria more. "If I hadn't left home I wouldn't have come to see what is possible. If you're enmeshed in mediocrity, you just don't know how mediocre it all is. And because I know the potential in Nigeria, whenever I go back I think we could do better. Because I've seen how it's not that hard."

Since leaving, she has began to see what she calls the Nigerian swagger – the attitude that causes resentment in other African countries. "We're not popular in any part of Africa. And we're rather proud of it. If I wasn't Nigerian, I think I would understand why. There's a kind of Nigerian aggressiveness … 'Why shouldn't we?' We'll do it very loudly and without much finesse, but hey. Inside Nigeria there are different cultures, but this is Nigerianness – it cuts across ethnic groups. I don't know if it's from our large size, I don't know if it's because we never had white people settle and stay. So Nigerians go to Kenya and Tanzania and we think, why are you so apologetic?"

Adichie is quite up for a fight if one comes along. A moderator on a panel recently infelicitously called Americanah "a Nigerian Gone With the Wind", and she just about managed not to fly out of her seat, but said coldly: "I hope it's better than that." The love-story element is something she feels is often undervalued.

"Don't we all in the end write about love? All literature is about love. When men do it, it's a political comment on human relations. When women do it, it's just a love story. So, although I wanted to do much more than a love story, a part of me wants to push back against the idea that love stories are not important. I wanted to use a love story to talk about other things. But really in the end, it's just a love story."