Barbara Ehrenreich has been called a Marxist just for writing that the US is not a classless society. But criticism has never stopped her exposing social injustice before. Emma Brockes talks to her about her new book, Barack Obama and the great wealth divide

Twenty years ago, Barbara Ehrenreich wrote an article for the New York Times in which she pointed out the growing inequality of American society and was promptly denounced, by a rival paper, as a Marxist. "The Washington Times is an extreme-rightwing publication," she says, so there was no surprise there. But the paper's reaction underlined a general principle: that while one can say "fairly wild" things about race and gender in the US, there persists a certain coyness about class. "There's this powerful myth that America doesn't have classes; that they're an ancient English or European thing that we abolished. And that if you're not rich, it's your own damn fault."

Now 66, Ehrenreich has devoted most of her career to disproving this maxim. Her 2001 bestseller Nickel and Dimed was an account of the year she spent trying to eke out an existence on the minimum wage, which caused affluent readers everywhere to exclaim guiltily: "We had no idea!" She reported that companies cheat their staff of wages (there are 70 lawsuits pending); limit the number of toilet breaks staff take; forbid them from talking to each other or using "profanity" on the premises, and that the cleaner you hired through a "reputable" firm is probably made to clean your house while sick or injured. The book's success owed much to the personal journey of Ehrenreich herself, who suggested the idea to her editor for a younger journalist to take on. But she fitted the profile of the invisible worker - middle-aged, female and knackered. Once in situ, she was bullied by various bosses and forced to retire each night to a motel because she couldn't afford a flat.

Her latest book, which in the US is called This Land Is Their Land: Reports from a Divided Nation, is the animating force behind all this, a collection of columns that almost amounts to a manifesto. The title comes from a Woody Guthrie song, which Ehrenreich can hardly bear to listen to these days. She writes: "I flinch when I hear Woody Guthrie's line, 'This land belongs to you and me'. Somehow, I don't think it was meant to be sung by a chorus of hedge-fund operators." (The book's UK publisher evidently didn't feel Guthrie's song travelled well, and has opted for the title Going to Extremes instead.)

Ehrenreich's skill, apart from the sheer quality of her writing, is to illustrate her opinions with wave after wave of examples, of unglamorous labour disputes and everyday injustices that don't get much of a look-in elsewhere. Through them she details how wealth in America has transferred from the bottom to the top, thanks to tax cuts for the rich and Bush's reluctance to regulate the markets, and exposes the fallacy that "growth" as measured by GDP is, for the majority of Americans, synonymous with better living.

"It was just so fascinating to me, without being an economist, to see how in the past few years growth has become completely decoupled from wages or the real conditions of what we call working people," she says. "And the reason they were so decoupled is because of the huge inequality. So you could have many [economic] indicators looking very sunny and good, but you're talking about a population that is so divided there's not an average there any more."

A book about the joylessness of the American right must struggle to avoid matching it with a litany of dreary, rival orthodoxies. But Ehrenreich has never been dour, nor for that matter predictable. She lives in the historic town of Alexandria, just south of Washington DC, in a jolly chaos of papers and magazines. On the mantelpiece is a card that reads, "I am not, therefore I buy", but she is as suspicious of self-denial as she is of self-indulgence, both of which she sees as affectations. In one unexpected column, Ehrenreich flies at Jane Brody, the health editor of the New York Times, who throughout the 90s championed with great influence the virtues of a low-fat, high-carb diet. As well as questioning the health benefits of Brody's principles, Ehrenreich calls them a way of enabling the well-off to feel virtuous merely by indulging their own narcissism. "The low-fat diet has been the hair shirt under the fur coat - the daily deprivation that offsets the endless greed."

The "tireless preaching" that bedevils modern life elicits a resounding screw-you from Ehrenreich. Her latest bugbear is "positive thinking", the underlying philosophy of much life coaching and motivational speaking, which she came across during the research for Bait and Switch, the follow-up to Nickel and Dimed. In it, she spent a year trying to expose white-collar office life but was scuppered by not being able to get a job. Instead Ehrenreich fell into the hands of the gannets who feed on the unemployed and sell them reassurances that getting a job is just a question of attitude. This was illustrated by cheerful Kimberly, a "co-active coach" whom Ehrenreich employed and ended up wanting to kill. As the economy recedes, you wonder if Kimberly and her ilk will disappear. "I tend to think that the irrational, delusional approaches will persist," she warns.

Ehrenreich is by training a scientist, with a degree in chemistry and a PhD in cell biology. As a child she saw both sides of the economic divide. Her father was a copper miner from Montana who got an education and eventually qualified as a metallurgist and made it on to the corporate ladder at Gillette. "He was a very exceptional person, as he'd be the first to tell you. But he never - nor did my mother - say about people who didn't do as well, 'Oh we did it, so they can do it.' They recognised that theirs was an unusual trajectory."

Did they identify as working-class?

"No. I think they would have said middle-class. But I think my father always thought that he didn't fit in. He was too rough-edged. And he had a lot of contempt for, say, Ivy League types or MBA types."

What she sees as the stigmatisation of the sick in the US is a reaction in part to a "strange little detail" of her childhood. Her mother, who was politically more radical than her father and whom the young Ehrenreich would look at in alarm sometimes and wonder if she was a communist, had been brought up by her Christian Scientist grandparents. "And in no other way was my mother continuing to be a Christian Scientist, except for one thing: health. It was very bad to get sick. I remember when I had trouble seeing the blackboard in about seventh grade, she said, "People in our family don't wear glasses." Ehrenreich smiles ruefully.

Her son is a writer and her daughter a lawyer, (Ehrenreich is divorced; she moved to Alexandria to be near her two grandchildren) and half of her family still lives on low wages; her sister and her husband have just been forced to cancel their health insurance. I wonder if she had ethical qualms about Nickel and Dimed; isn't there something unsavoury about a comfortable journalist pretending to be poor and then being paid a lot of money to write about it?

"Well you know, that never entered my mind . . . what began to bother me a little bit was that there was a deception involved; that I had to tell people that I was working these jobs because I needed the money, which wasn't true. But I always tried at the end to tell people I had got to know what the truth was. And then you can work off the guilt of any money by giving it away. Easily fixed."

Until the success of that book she had been freelance, and the security, she says, has been wonderful. She hasn't had a staff position since her first job working for the New York City government as a health planner, which she left after seven months when she decided that "the government was selling out to private interests" and went to work for a "radical collective" lobbying for better healthcare in the city. "That's where I started writing, because we had a newsletter and I loved to do investigative pieces."

Nowadays, people write to Ehrenreich with their workplace horror stories. The most shocking in the new book came from an ex-employee of one large retailer, who told Ehrenreich that in 2003 the company held him captive for six hours and interrogated him for giving a colleague a discount on a videogame, before getting him to write a false confession and firing him. A former colleague alleged that such incidents were not unusual.

With Obama ascending there is hope of a sea change, although Ehrenreich remains characteristically cautious. She sees him "tacking to the right" and was disheartened by his choice of economic adviser, Jason Furman, "who was to the far right of the Democratic party and made his reputation as a defender of Wal-Mart [one of her principal targets in Nickel and Dimed]. And so in a way, I thought, OK, I'm not going to pay [Obama] any attention for a while."

I wonder if the huge success of Nickel and Dimed, and the tax bill that presumably came with it, hasn't sent Ehrenreich skidding off a bit in that direction. "Ha! I have to watch that kind of stuff. But no. I always say, if I could pay more taxes and be in turn told for sure that there would be decent schools for my grandchildren, that there would be healthcare for them, that there would be social security, if there was something in return, other than wars, it would be a wonderful thing." She cackles. "As it is, I just get angrier and angrier"

· Going to Extremes: Notes From A Divided Nation is published by Granta (£8.99).To order a copy for £8.99 with free UK p&p go to theguardian.com/bookshop or call 0870 836 0875