There are some novels that tell a great story and others that make you change the way you look at the world. Chimamanda Ngozi Adichie's Americanah is a book that manages to do both.

It is ostensibly a love story  the tale of childhood sweethearts at school in Nigeria whose lives take different paths when they seek their fortunes in America and England  but it is also a brilliant dissection of modern attitudes to race, spanning three continents and touching on issues of identity, loss and loneliness.

This is Adichie's third and most ambitious novel  her first, Purple Hibiscus, was longlisted for the Booker prize and her second, Half a Yellow Sun, won the Orange prize. A highly acclaimed 2009 collection of short stories, The Thing Around Your Neck, cemented her position as one of the most promising African writers of her generation. She was awarded a prestigious MacArthur "Genius" grant and in 2010, the New Yorker featured her in its list of the 20 best authors under the age of 40.

So a lot is expected of her. Gratifyingly, Americanah does not disappoint. It tells the story of Ifemelu, a spirited young girl with strong opinions, and her teenage boyfriend, Obinze, who grow up with romanticised notions of the west, shaped by the literature of Graham Greene, Mark Twain and James Baldwin. When Ifemelu is presented with an opportunity to continue her postgraduate studies in Philadelphia, she takes it. Some years later, Obinze, too, goes in search of a better life, but to Britain.

It is at this point that Adichie really begins to flex her muscles as a novelist: the sense of dislocation felt by both characters in two countries with wholly different histories and class structures is expertly rendered. She has an extraordinary eye for the telling nuance of social interaction within a particular kind of liberal elite.

In England, Obinze struggles to get hold of the ever-elusive national security number that will enable him to work legally. The newspapers are full of stories about schools "swamped" by immigrant children and politicians' attempts to clamp down on asylum seekers. Against this backdrop, he is invited to a smug Islington lunch party by Emenike, a former classmate in Nigeria, who has married a high-flying solicitor. The food is served on self-consciously "ethnic" plates brought back from a holiday in India and Obinze is left wondering whether Emenike has become a person "who believed that something was beautiful because it was handmade by poor people in a foreign country, or whether he had simply learned to pretend so".

The polite conversation skates over race and the idea of foreignness, with each guest trying to outdo the next with their earnest political correctness. Adichie skewers their self-satisfaction with lethal accuracy. They "understood the fleeing from war, from the kind of poverty that crushed human souls", she writes. "They would not understand why people like him, who were raised well-fed and watered but mired in dissatisfaction, conditioned from birth to look towards somewhere else were now resolved to do dangerous things, illegal things, so as to leave, none of them starving, or raped, or from burned villages, but merely hungry for choice and certainty."

In America, Ifemelu also finds it difficult to get part-time work. She gets turned away from menial jobs as a waitress, bartender or cashier. Her fellow students speak to her with painful slowness, as if she cannot comprehend basic English. In class, she is singled out as someone who will intuitively understand the plight of African Americans because of some half-formed belief in a nebulous, shared "black" consciousness.

Adichie is particularly good at exposing the contradictory ebb and flow of America's painful attempts to reconcile itself with its recent past, when segregation still persisted in the south. She does so with a wryness and insight that never imposes itself on the flow of the story but which challenges the reader's assumptions with each carefully crafted sentence.

There is the blond, well-heeled Kimberley who means well but says every black woman she sees is "beautiful", despite aesthetic evidence to the contrary. When Ifemelu buys a vintage 1960s dress on eBay she realises that when the original owner would have worn it, black Americans would not have been allowed to vote.

"And maybe," Ifemelu notes, "the original owner was one of those women, in the famous sepia photographs, standing by in hordes outside schools shouting 'Ape!' at young black children because they did not want them to go to school with their young white children."

Eventually, Ifemelu starts blogging about her experiences. Adichie captures the tone of internet chatter with precision  at once both breezy and sporadically furious  and the blogposts add an extra dimension to the plot, allowing the reader to see how Ifemelu sees herself and how she wishes to present herself to the outside world.

A recurring theme of the blogs is the politics of black hair  how women are expected to relax their natural curls with toxic chemicals or weave in bits of someone else's hair in order to conform to comfortable white norms. In fact, much of the novel is written in flashback, as Ifemelu has her hair braided in a New Jersey salon in preparation for going home to Nigeria after 15 years in America, during which she has witnessed Barack Obama's election victory.

The final section of the book follows Ifemelu's return and her reunion with Obinze who is, by now, married to someone else. It is to Adichie's immense credit that such a sprawling, epic book remains so tightly structured. There are, perhaps, one too many of Ifemelu's blogposts and a few extra scenes here and there that could have been cut, but part of Americanah's appeal is its immense, uncontained and beating heart. You can feel Adichie's passion and belief pumping beneath every paragraph.

Americanah is a deeply felt book, written with equal parts lyricism and erudition. More than that, it is an important book  and yet one that never lets its importance weigh down the need to tell a truly gripping human story.

After 13 years in the United States, Ifemelu is about to return to Lagos; but first she must go to the hairdresser's. So far, so run-of-the-mill, for who doesn't want to look their best to greet a crowd of people they haven't seen for a long time? But for Ifemelu, this essential piece of personal maintenance is not exactly straightforward. First, she must take a train out of Princeton, where the few black people she has seen are "so light-skinned and lank-haired she could not imagine them wearing braids", then she must take a cab to an unfamiliar salon, her usual hairdresser being unavailable because she has returned to Ivory Coast to get married; then wrangle over the price; then sit in baking heat for many hours, during which she will be asked repeatedly whether she knows the Nollywood stars on the television and, more alarmingly, whether she can intercede on her Senegalese braider Aisha's behalf to persuade either of her Igbo suitors to marry her.

Hair is a big deal in Americanah (the slang term that Ifemelu's Lagos friends will use to describe her when she goes back to Nigeria). "Why don't you have relaxer?" asks Aisha, to which she replies, "I like my hair the way God made it", meaning that she refuses to straighten her hair by means of chemicals and smoothing irons; but it is also a statement made ironic by its context, given that the pair are in the midst of a disagreement about what colour hair extensions Aisha should use to weave into Ifemelu's braids. "Colour one is too black, it looks fake," Ifemelu tells her, but Aisha merely "shrugged, a haughty shrug, as though it was not her problem if her customer did not have good taste".

What is real, what is fake, how many layers of history and culture it takes to construct a national, or racial, or personal identity, and how contingent that identity is on its immediate surroundings are all questions that Chimamanda Ngozi Adichie poses in her third novel; but her real talent is to make those questions seem as if they cannot be contained by neat, orderly language, and instead to animate them, to embed them in messy, difficult lives that are filled with idiosyncrasy and complication and compromise.

Ifemelu has herself created a life based on observing the weirdnesses  mostly painful, sometimes comical  that emerge when different groups of people live together in a system shaped to maintain the dominance of one group over others. Her blog, Raceteenth or Various Observations About American Blacks (Those Formerly Known As Negroes) by a Non- American Black, created so that  she could voice her various puzzlements and conclusions about what she saw around her, has become a huge success, managing to keep happy both the kind of readers who routinely use the word "reify" and those who want to chat in more laidback fashion about their experiences. In posts such as Badly Dressed White Middle Managers From Ohio Are Not Always What You Think, about a man who has adopted a black child and finds himself shunned by his neighbours, she chronicles her unexpected discoveries; in more didactic mode, she counsels her fellow immigrants in unabashedly straightforward, no-nonsense terms. Stop telling Americans you are Jamaican or Ghanaian, she writes in To My Fellow Non-American Black: In America, You Are Black, Baby, because "America doesn't care": "You must nod back when a black person nods at you in a heavily white area. It is called the black nod  If you go to eat in a restaurant, please tip generously. Otherwise the next black person who comes in will get awful service, because waiters groan when they get a black table. You see, black people have a gene that makes them not tip, so please overpower that gene."

In the process, Ifemelu has gone from being broke, depressed and alienated to being a condo-owning Fellow at Princeton. She would not wish to return to her early student life in America, when she was forced to help a sports coach to "relax" so that she could pay her rent; when she was utterly bewildered by the customs of the country. But nor is she quite at home with her life as it is; and a kind of weariness, a build-up of "amorphous longings, shapeless desires" has led her to this point of departure.

There are also more concrete reasons: perhaps the example of her Aunty Uju, a doctor who came to America following the death of the military high-up who kept her in fine style as his mistress, but who has found herself incrementally diminished by it; or Ifemelu's failure to find a definitively comfortable fit with her painstakingly moral and politically fastidious boyfriend Blaine; or by the knowledge that she herself feels a disconnect in what she is doing. "You know why Ifemelu can write that blog, by the way?" asks Shan, Blaine's jealous and unpleasant sister. "Because she's African. She's writing from the outside. She doesn't really feel all the stuff she's writing about. It's all quaint and curious to her. So she can write it and get all these accolades and get invited to give talks. If she were African-American, she'd just be labelled angry and shunned." The tension between these two characters has simmered for some time, and this is an explosive moment. But Ifemelu barely reacts, saying only "I think that's fair".

And there is also Obinze, the childhood sweetheart  indeed, once her future husband  whom she left in Nigeria and who shares, as a lesser partner, the narrative. Obinze's experience of emigration has been less successful than Ifemelu's; a brief stint in London sees him working under a false name and paying over the odds for an arranged marriage, only to be arrested on his way to the ceremony and later deported from a country "odorous with fear of asylum seekers". He has also seen friends from home in decidedly elevated circumstances: Emenike, who has married a wealthy lawyer and subsequently "cast home as the jungle and himself as interpreter of the jungle", invites him to a dinner party in Islington, at which Obinze is struck by the unmatched artisan plates that would never be used for guests in Nigeria. More unbridgeable, though, is his fellow guests' inability to understand he is not a refugee: "They would not understand why people like him, who were raised well-fed and watered but mired in dissatisfaction, conditioned from birth to look towards somewhere else and eternally convinced that real lives happened in that somewhere else, were now resolved to do dangerous things, illegal things, so as to leave, none of them starving, or raped, or from burned villages, but merely hungry for choice and certainty."

Obinze's enforced return to Nigeria brings power, albeit through chance connections, so that when, towards the end of the book, Ifemelu arrives in Lagos as an awkward outsider, he is very much part of the new establishment. Whether they are able to retrieve their former intimacy, or whether it has been chased away by the transformations wrought in them by their travels, provides a tentative resolution.

But it is also slightly unsatisfactory, because Americanah is a book that works better when it is in transit, detailing people and situations who are in the act of becoming. Its structure is complex and sometimes unwieldy; there is much looping backwards and forwards in time as Ifemelu sits in the hair salon, and one feels slightly lost once her braids are finished and the narrative has moved on. Similarly, some characters are glimpsed too fleetingly to make a lasting impression; in the case of Ifemelu's parents, for example, this neatly mirrors their daughter's fading memories of them, but it is also tricky for the reader.

Nonetheless, this is an impressive novel  although very different from Adichie's Orange prize-winning Half of a Yellow Sun, it shares some of its freewheeling, zesty expansiveness. But that should not disguise its delicacy; it is also an extremely thoughtful, subtly provocative exploration of structural inequality, of different kinds of oppression, of gender roles, of the idea of home. Subtle, but not afraid to pull its punches. We all wish race was not an issue, says Ifemelu, talking about inter-racial relationships at a polite Manhattan dinner party, the day after Obama becomes the presidential candidate: "But it's a lie. I came from a country where race was not an issue, I did not think of myself as black and I only became black when I came to America."