Mira Kirshenbaum swings open the door to her penthouse overlooking the lights of Los Angeles. "Come in," she purrs, dark eyes twinkling in the night. Her hand brushes my arm. "You know," she breathes, "good people have affairs too."

Actually, that's not how it went at all. But the attention garnered by Kirshenbam's latest self-help book, When Good People Have Affairs, would have us believe that she is a vampish incarnation of the moustache-twirling lothario of old. An agent sent to sow seeds of unrest among the happily married, she arrives, we learn, with a message that infidelity is fine, and that the only bad adulterer is the one who owns up.

But the message, like the person, has become confused in the telling. The real Mira Kirshenbaum is something altogether different from the media image. On a bright, hot Saturday afternoon she swings open the wooden gate to her daughter's hillside house, close to downtown LA. Wearing a dark blue T-shirt and slacks, she looks more like someone's sixtysomething granny than the latest threat to the peace and harmony of civilised society. Actually, she is somebody's granny. "My throat is sore," she explains. "I've been singing lullabies."

We sit on a wooden deck sipping water while her two-year-old grandson has an afternoon nap. It is all achingly normal. But for Kirshenbaum, the reaction to her work has been anything but normal. Obviously reeling from the assault, she is a therapist in need of therapy. Kirshenbaum is delicate of bearing and, seemingly, delicate of sensibility. Her eyes and high forehead bear out her Slavic roots, while her softly spoken voice suggests not just the therapist but the lilt of English learned as a second language.

"The BBC gave out my number to their radio-show producers and people were calling me up day and night, putting me on the air and yelling at me," she says. "It's not just England.

Ireland, India, Canada. And Edinburgh. I couldn't believe the woman in Edinburgh. She wrote a piece telling all the people to come and beat me up. She hadn't read the book. Anyone who had read the book would not be saying these things."

The article in the Edinburgh Evening News was certainly forthright. "I'll happily confess that I have no anger issues," wrote Sarah Howden, before becoming extremely angry after reading about Kirshenbaum's book. "I could have jumped on a plane, hunted her down and beaten her with my bare fists, turning all my hurt of being cheated on in the past into powerful blows."

Others have taken the nudge-nudge, wink-wink approach to the thorny issue of infidelity: "Why having an affair could save your marriage," giggled one newspaper; "Thou shalt commit adultery, just don't tell," nudged another; "Why adultery can help save a marriage," a third. It is a headline-writer's dream. Kirshenbaum is dumbfounded by all the attention.

"It would never occur to me that this would happen," she says. "We were afraid of the brown-bag effect, so the publisher put the title in pink letters. They did everything possible to not draw attention to the book - and then it exploded. It was unbelievable."

When Good People Have Affairs is Kirshenbaum's 11th self-help book. She came to therapy in the early 70s, after having studied under most of the grand names of the practice. "It was very creative," she says of therapy in the late 60s and early 70s. "I went directly to the people who were the masters in the field, I read their books and I started studying with them. I learned to imitate the style of everyone I was studying with."

Kirshenbaum was born in Uzbekistan in 1945. Her Jewish parents survived because her mother, who spoke good Russian and Polish, was able to pass as a gentile. The family spent the first four years of her life in a displaced persons' camp in Germany before her parents separated, her father going with her older sister to live in Israel, Kirshenbaum coming with her mother and stepbrother to the US. The absence of a father during her early childhood weighed heavily. "My mother was a single mother and I saw all the children had a father and I told my mother from morning to night, I want a papa, I want a papa. And she got one. I chose him for her."

At the age of 16, she decided to go to Israel to look for her biological father. "And I found him," she says. "I knocked on his door and said, 'I'm your daughter.'"

The temptation to stay in Israel with her father was strong but she decided to return to the US to study, taking courses in philosophy in New York, before moving on to postgraduate studies in phenomenological psychology.

Common to the genre, When Good People Have Affairs has the affliction of a lengthy title - Inside the Hearts & Minds of People in Two Relationships, reads the subtitle. Her other works include Too Good to Leave, Too Bad to Stay; Our Love is Too Good to Feel So Bad; The Weekend Marriage: Abundant Love in a Time-Starved World; and Everything Happens For a Reason.

When Good People Have Affairs is simple, concise and full of what to some may be blindingly obvious and to others could be the insight necessary to change their lives. Literature it isn't, and for readers not involved in two relationships, the plain, often repetitive prose can be akin to reading a shopping catalogue. It includes a list of 17 motives for affairs, ranging from the "see-if affair" to the "midmarriage-crisis affair", via the hyphen-crazed "let's-kill-this-relationship-and-see-if-it-comes-back-to-life affair".

The book's core lies in understanding the motives for an affair, and using that information to sort out what the "cheater" - to use the genre's lexicon - may have been wanting.

"They have to regret-proof their decision," says Kirshenbaum. "The whole approach is breaking things down into small steps so that at every point they're making the best decision they can make given the information they have."

And she is keen to clarify that she is talking about "good people - people who don't want to hurt anyone and when they do realise that they have hurt someone are truly sorry, and paralysed. That's the emotional state they are in." Conversely, the 15-20% who have affairs and don't care who they hurt, are "sociopaths".

Kirshenbaum insists that she is one of the good people, contrary to what her detractors would have us believe. Her peers, she says, told her not to write the book, arguing that she had overstepped the boundaries of the practice.

"You're supposed to help people stay together," she says. "You're not supposed to say, you know, some problems can't be fixed and if they can't be fixed, then you should look for your happiness somewhere else. You're not allowed to say that to people as a therapist and I did, because I promised myself as a kid that I would always tell the truth, about love and about everything. I felt that was what I had to do, otherwise it was pointless to write books if I wasn't going to tell the truth."

One of the most contentious points in the book is the advice that being honest is not always the best policy, that more harm than good may come of the unburdening of misdeeds. The advice comes from her own experience.

"My husband explained to me," she begins, her eyes watering. "We'd been married for over 10 years - this was about 30 years ago - and I was very busy, I was a therapist, we had small children." She pauses. "He got to know this woman and they became very, very close and he confessed it to me. I was just devastated. I felt the whole past, the whole meaning of our lives, of everything, was shifted for me. For years and years I just wanted to know details. It was horrible. It kept me stuck in the past. We just fought about it and were miserable. I knew that he was a good man and I wish I hadn't known."

Her features crumple as she stares back at her memories. Her husband, her partner and co-founder of the Chestnut Hill Institute in Boston, where she practises, is, she insists, a good man, the sort of man you might find in her book. And like all good men, she says, he recoils from inflicting suffering. "My husband would not do it again," she says. "I would bet on that."

Many of the critics of Kirshenbaum's latest treatise have not read her book. Although available on the internet, it does not have a British publisher. "I'm going to get into trouble for saying this," says Kirshenbaum, "but my agent told me he was unable to sell the book in England.

I hope he can sell the book now, because a lot of people need this. And the fact that there's been such an uproar tells me that I hit a nerve."

So does the old adage that all publicity is good publicity hold true?

"I don't know if I'm going to be allowed to write another book after this," she says. "It's more publicity than I've ever had. But the problem is they mentally deleted the When, and they read it as Good People Have Affairs, as if I were legislating or issuing an injunction. I'm a clinician. I'm not a moralist, I'm not a politician."

But Kirshenbaum is, perhaps, a romantic. Her parents separated, she urged her mother to remarry, she has been in the same marriage for more than 40 years, and she has devoted her professional life to helping couples get through the bad times. "I'm not a slut," she says, responding to one of the printed verdicts about her. "They just made assumptions about who I was"