After finishing Patricia Cornwell’s novel Postmortem, about a serial killer who stalks and slashes lone women, I wander down to the basement gym of my New York hotel. Usually I’d be pleased to find it empty, but instead I prickle with unease: I imagine some masked psycho bursting in, dumbbells and skipping ropes plied as weapons, my toe tagged, my innards pored over by Cornwell’s pathologist heroine, Dr Kay Scarpetta. Freaked out, I scurry back to my room.

When I tell Cornwell this she is delighted. “That’s good!” she exclaims. “If ever you don’t feel comfortable, you should trust your gut. There have been great studies of victims who survive and they all say the same thing, ‘I got a funny feeling and I didn’t listen.’” What I don’t say is that I’ve used that gym alone before, untroubled; it was Cornwell’s unblinking brand of rape-homicide fiction – a woman is almost decapitated in Body of Evidence, peeled in From Potter’s Field, “water-boarded” in Book of the Dead – which got me, well, a bit paranoid.

But you wouldn’t use the P-word with Patricia Cornwell. Perhaps when you have attended hundreds of postmortems and crime scenes, spent the past 20 years immersed in the spatter, reek and gore of forensic science, and turned it into 20 bestselling novels and an estimated $100 million fortune, you evaluate risk differently. The imagination that made her the most successful crime writer in the world forbids her to take chances. Today, her bodyguard, an ex-Marine called Jimmy, waits outside in a black Porsche Cayenne. Even when walking her dogs with a friend, she takes one of her several handguns: “What if a bunch of drunk guys stopped their car?” Her public appearances are policed by hard-bodied men whispering into their cuffs, and her home, near Cambridge, Massachusetts, has elaborate camera systems and a permanent security detail: “There are a lot of wacky people out there and it only takes one…”

Cornwell’s friends call her Mrs Worst-Case Scenario, but she insists: “What I worry about is legitimate.” So if, say, she is renting a house by the ocean and has the balcony door open she’ll barricade it with a chair: “I’m convinced that lots of people who were supposed to have committed suicide off a balcony were really accidents.”

In person, she shows the strain of living on perpetual high alert. She is watchful and intense, seldom smiling, but then suddenly gives a magnificently frank and generous answer she must pore over regretfully later, given the fusillade of e-mails I receive from her agent’s office and then Cornwell herself, fretful of misinterpretation. She gives the impression of barely containing many powerful, competing emotions within a very thin skin.

The interview had been set for New York, but within an hour of arriving I got a call saying that she had a fever of 102F, and wouldn’t be chartering a Learjet down from Boston after all. Usually with American big shots, this means a doomed trip or at least a thumb-twiddling week until the star rallies. But Cornwell, being both tough and kind, agrees to dose herself on flu meds and meet me nearer her home, at the Harvard Faculty Club, a grand and sumptuous building filled with antiques, burnished silver, trompe l’oeil and dark, polished wood; the place where vast wealth and academia meet.

We are only allowed here at all because her partner, Dr Staci Gruber, whom she married three years ago (and to whom Book of the Dead is dedicated), is assistant professor of psychiatry. Indeed the faculty club, with its rigid exclusivity, is a leitmotiv in her new book, The Front. It is where her new hero, the lowly but virtuous, second-hand-suited state investigator Win Garano, is summoned to take orders from his manipulative district attorney boss, Monique Lamont. “Win doesn’t belong here,” Cornwell says. “He has a learning disability and he could never have gotten into Harvard, yet he’s smart enough he could have gone anywhere.” Likewise, for all her success and wealth, and the fact that two blocks away is the multi-million-dollar collection of Walter Sickert artworks she donated to Harvard’s Fogg Art Museum, Cornwell is also an outsider, a status she seems both to resent and relish. “I tried to get into grad school here and they turned me down,” she says. “I came from a little town in the mountains of North Carolina and I’d never heard of anyone who went to Harvard. I’m around people who are doctors and lawyers and scientists and forensic pathologists. I feel insecure about my education. It doesn’t make any difference in what I write, but it probably keeps me from being a snob. I have so many things wrong with me, and it’s probably a good thing or I’d be an asshole.”

Cornwell is the most image-aware of authors. Few interviews fail to mention her Armani suit and professional grooming. But today, straight from her sick bed, she has the kick-ass look of an off-duty FBI agent: navy combat pants, biker boots and a red body-warmer, which, like a chunky ring on her right hand, bears the Scarpetta insignia. “Oh, I’m a slob today. You get a rare experience.” She is lean, sinewy and toned, but her face has been tightened and evened by surgery, Botox and collagen until, without make-up at 51, she looks like a slightly freeze-dried Meg Ryan. “Listen, honey,” she says in her Carolina drawl, “my goal is to make sure before I die I won’t decompose. Sure I’ve had work on my face. I’ll get anything done I can! I don’t want to look old. Does anyone?”

This is the breezy pragmatism of the utterly self-made: she transformed her destiny, why stop at her face? Indeed, all Cornwell’s tough talk and her obsessive self-protection are products of a gothic childhood in which she learnt to take care of herself because no one else would. When she was five, her father, a lawyer, left her family for good, Cornwell clinging pitifully to his legs. Her mother tumbled into despair and then mental illness. Cornwell seems unforgiving, even disgusted by this weakness. She recalls as a young child being molested by a private security guard near her home. “And the next thing I know is there is a police officer at my house and I’m at some kind of hearing at the court house and strangers are passing my little red shorts around, the ones he’d put his hands inside. I feel fear and that what I did was bad. My mother’s only way to deal with it is to take me to a toy store afterwards. And never to talk about what happened except to tell me I can’t ever buy a pair of red shorts because it will give [me] bad memories. Why not just say nothing?”

When Cornwell’s mother needed to be hospitalised she drove Cornwell and her two brothers to the nearby home of preacher Billy Graham. His wife, Ruth, welcomed them in and found foster care, even though the family were strangers. Ruth Graham remained Cornwell’s beloved mentor until her death last June, sending her cheques at college, guiding her through adolescent anorexia and, above all, encouraging her to write. But Cornwell’s foster mother, a missionary, terrified Cornwell, forbade her to leave the house, tormented her, fed her food she found disgusting. “I probably kill this lady every time I write a book,” she says grimly. “I find some way or form. She’s dead now and she deserves it.”

Cornwell never read a single murder mystery while growing up, but she was drawn to the macabre: her first poem, an ode to Abraham Lincoln’s assassination, lingered on his wounds. She believes it is her own fear of loss that underpins her work. Her interest in crime was only ignited after college when she found a reporting job on The Charlotte Observer and was assigned the police beat. “I was seeing car accidents and murders. That is what infected me with crime. I was so horrified I tried to figure it out.”

So she took a job as a data programmer at a morgue. Forensics, the science of reconstructing for court a whole life from a fibre, a fleck of paint, a nick on bone, echoed her childhood passion for archaeology. The morgue was run by a woman medical examiner, whom Cornwell begged to let her attend an autopsy. She even became a volunteer policewoman every weekend for three years to win permission. “I viewed it as a clinical scientific experience,” she says. “I tried not to focus on the gore or the dead body part of it.”

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