Of all the mantles that have been foisted on Toni Morrison's shoulders, the heaviest has to be "the conscience of America". It's both absurd-sounding and true. For almost half a century her subject has been racial prejudice in the United States, a story that she has told and retold with a steadiness of rage and compassion. Her latest novel, God Help the Child, is her 11th and when I arrive at her apartment in Tribeca, Lower Manhattan, America's Conscience is having her eyebrows drawn on. "For the photographer," she explains with a chuckle.

Later, she'll tell the photographer: "We did makeup for you. I have eyebrows and everything," then add: "You lose all that stuff ... " The implied second half of that sentence is "when you reach my age": Morrison turned 84 in February. Her many literary laurels include a Pulitzer in 1988 for , a Nobel in 1993, and, in 2012, the presidential medal of freedom, from her friend Barack Obama. Being America's most venerated living writer does not, however, stop a person wanting to look good in pictures. And, it is natural that beauty and the notion of self-image are on her mind as at the centre of her new book is a striking, dark-skinned woman called Bride who tries to shield herself from her own past with surface beautification. A love story unfolds, precariously, between her and Booker, a scholarly young black man adrift in grief for a dead brother. He tells her: "scientifically there's no such thing as race, Bride, so racism without race is a choice. Taught, of course, by those who need it, but still a choice. Folks who practice it would be nothing without it."

Bride's blackness is both the source of her childhood misery - her lighter-skinned mother is so horrified by it that she considers killing her baby - and of her adult success. She works in the fashion and beauty industry where, heeding one stylist's dictum to dress only in white, she makes herself, "a panther in snow", an exoticised "other". The novel intimates that fetishising blackness, both for the observer and the observed, might be just as insidious as outright prejudice. There's the ex-boyfriend, for example, who seems to claim her as some kind of racial trophy. When this young white man takes her home to his parents it's clear "that I was there to terrorise his family, a means of threat to this nice old white couple. 'Isn't she beautiful?' he kept repeating ... His eyes were gleaming with malice."

"I'm trying to say," Morrison tells me now, "it's just a colour."

As for beauty: "It can destabilise you if that's all you have and that's all you care about and that's where your success comes from. There's a three-dimensional person somewhere outside the clothes and the makeup and the nudity, as they call it, since everybody beautiful is buck naked now. I mean," she says, switching into a tone of outrage that is tinged with self-parody - an older woman pronouncing on the waywardness of the young - "they don't even make gowns any more that are not, you know ..." and she gestures over her bosom to delineate extreme skimpiness.

"Now think about this," she continues, her voice becoming low and mysterious in the manner of a seasoned storyteller. She pauses for effect. "The nipple is the first thing every human being sucks on. Comfort, nurture, you know? But it's not like 'Uhh'" and she mimes jutting a breast out in sexual exaggeration. Once her wheezes of laughter subside, she observes mildly: "That's interesting how that happened."

The new novel's obvious precedent is 1981's , the only other of her novels to have a contemporary setting, in which a Sorbonne-educated fashion model, Jadine, who fears she has been deracinated by the world of white culture she has come to inhabit, falls in love with Son, a penniless drifter at complete ease with himself and his blackness. If more seems to be at stake in this earlier book, it might simply be a reflection of the increasing superficiality of our moment: Jadine may have been a model but she is not the appearance-obsessed, emotionally stunted child-woman that Bride is. The universe of God Help the Child can seem a little thinner, even as redemption and deliverance bloom.

But with its island of spirits and talking trees, Tar Baby, Morrison points out, is more timeless phantasmagoria than identifiable present reality. So this, really, is her first contemporary novel and she admits that it gave her some trepidation. "It was so fluid," she says. "Everything else I sort of had a theme about but this doesn't have any anchor for me. But then I thought, well, yes it does, it's what we started this conversation about. Beauty - and its worth in the world. And what does that do."

It was a similar question that began her publishing career 45 years ago. She has always talked about her first novel with disarming simplicity: it was the book she wanted to read and that did not exist. So, as a single working mother of two small sons, she rose at 4am every day and wrote it. Published in 1970, The Bluest Eye is the story of Pecola Breedlove, a young black girl who prays for blue eyes. Morrison wrote in a 2007 foreword that she wanted to focus "on how something as grotesque as the demonisation of an entire race could take root inside the most delicate member of society: a child; the most vulnerable member: a female".

Most writers claim to abhor labels but Morrison has always welcomed the term "black writer". "I'm writing for black people," she says, "in the same way that Tolstoy was not writing for me, a 14-year-old coloured girl from Lorain, Ohio. I don't have to apologise or consider myself limited because I don't [write about white people] - which is not absolutely true, there are lots of white people in my books. The point is not having the white critic sit on your shoulder and approve it" - she refers to the writer James Baldwin talking about "a little white man deep inside of all of us". Did she exorcise hers? "Well I never really had it. I just never did."

She'd say it's because she grew up in Lorain, where the neighbourhood was racially mixed: Poles, Italians and Jews as well as African Americans. I'd say it's less to do with demographics and more to do with her own supreme self?assurance. It was this that propelled her to Howard University and then on to Cornell, where she completed a master's in literature. It was this self?assurance, too, that gave her the courage to split from her husband when she was pregnant with her second child and, that made her such an iconoclastic force as an editor at Random House where she propelled works by black writers such as Angela Davis and Toni Cade Bambara into the mainstream. Finally, of course, she herself became one of the publishing house's most cherished names.

It was Beloved, her 1987 novel about a slave woman who kills her own baby, that secured her current standing. When it failed to win the National book award, 48 black writers signed a letter of protest published in the New York Times. Soon after, it won the Pulitzer prize and a clutch of other awards.

In 1998, Oprah Winfrey produced and starred in a box-office-flop adaptation of the book and in the years since then, Morrison's literary reputation has been tainted with a slight suspicion of sentimentality, that snobby apprehension that she might be "a book club" author: the kind of writer, in other words, we read to feel better about ourselves, rather than the kind we read to better ourselves. Her novels, though, are not palliatives. There are moments in God Help the Child that made my stomach lurch with the same horror that I felt reading the description of Sethe in Beloved slitting her baby's throat. Evil itself, Morrison says, is, "completely boring": the thing she finds "intellectually fascinating" is how people respond to it. "Cowards are so dangerous. It's that quality that informs serious hostility - you want to kill somebody for whatever reason, or invent a reason, that's where that comes from."

This is the second time I have met Morrison. The first was three years ago, weeks after Trayvon Martin was killed. Then, she told me: "There are two things I want to see in life. One is a white kid shot in the back by a cop. Never happened. The second thing I want to see: a record of any white man in the entire history of the world who has been convicted of raping a black woman. Just one."

They are statements that attracted attention when she reiterated them recently. "Hasn't happened," she says of the first wish. "If it has happened, I don't know a thing about it. But that second thing? Never. Uh-uh. No matter what she says. No, not a white man. Even if everyone knows about it."

Arguably, there is yet to be "a good year for race in America", but 2014, with its sickening roster of black lives lost at the hands of police, seemed especially bleak. "You understand, don't you," she says, "that this is not new - it's in the press. Which is good but it's always been that way. I have sons. They have to say "Sir" if a police officer stops them. You know ... strategies for getting around."

The present tense and plural "sons" is poignant: Morrison lost Slade, her second son, to pancreatic cancer in 2010. His paintings - abstract portraits - hang on the wall opposite us, "all ears and no mouths because he said mouths are the most difficult thing". She pauses: "He died, somebody told me, five years ago. Is that true? I thought it was like two years ago, or maybe yesterday. How could it be five years ago? I don't know what to do with it."

There is bewilderment, too, when she considers the end of Obama's term. "Who's going to follow? I think his presidency's remarkable. I think it will be categorised as one of the most extraordinary, not only because of what he has done, but also because of the resistance." Her hopes rest on Hillary Clinton. "I respect and appreciate her. It was difficult in the beginning to choose between her and him. I didn't want to do the, 'Which is better? Gender or race?' And the only other thing I can say is the opposition is not even qualified. You know?" (Another great wheezing chuckle.) "There may be some people in the Democratic party who could give it a run. But, no, I would be on her side. Strongly."

A few months ago, when Morrison was interviewed by her friend Hilton Als, the writer and critic, she told him that now she's in her 80s, there are three things she gets to say. One is "No". The other is "Shut up". And the third is "Get out". In other words, she has earned her right not to do what she doesn't want to do. That includes writing a memoir, even though she signed a two?book contract with Random House that included one. "And then I thought about it and I said, 'I'm not writing a memoir: I'm not interested, I know that part'."

Later, she adds: "So much contemporary fiction, even when it's well written is sort of ... self-referential. I used to teach creative writing at Princeton and I would say ' Don't do that. Don't write about your little life'." Her tone is one of real distaste.

"Some people just close when they get old," she says. "But if you're open, if you have been, you can rely on the lived wisdom of the elderly. It's not the book learning, it's the lived wisdom. I ask friends of mine, 'How old are you, inside? ', and they always know. I know that I am 23. There's a moment when you just arrive." Or rather, in her case, there are many moments - at 84 going on 23, she continues to arrive.

God Help the Child is published by Chatto and Windus.