Katie Schottland, the protagonist in Susan Isaacs's new book, Past Perfect (Scribner, Feb. 13), calls herself a "Total Manhattan Sushi Woman." She grew up on New York City's Upper East Side, is Jewish, married to a WASP and, as the novel opens, is preparing to take her 10-year-old son to weight-loss camp. Katie is clever, funny, intrepid—a woman you want to root for. In short, she's the quintessential Isaacs character.

Isaacs knows how to write women you love to love, and she's got 10 bestselling books to prove it. From Judith Singer in Compromising Positions(1978), through Jane Cobleigh in Almost Paradise(1984), Linda Voss in Shining Through(1988), the title character in Lily White(1996), right up to Katie Schottland, Isaacs has defined herself as a champion of smart, plucky heroines.

Isaacs's books deliver. Her editor, Nan Graham, sums up their appeal: "She's so smart and so gutsy, and she's got an exuberance in her voice that's rare on the page." And book buyers agree: according to Nielsen BookScan, Isaacs's last two novels (2001's Long Time No See and 2004's Any Place I Hang My Hat) have sold more than 300,000 copies collectively. So why change a winning formula?

Yet the Long Island, N.Y., author consciously avoids the autopilot track: "I had a successful first novel..." she says. "But then what? Twenty-five years down the line I didn't want to be writing Judith Singer with Botox."

In Past Perfect, Isaacs revisits the CIA, a subject she touched on in Shining Through. The character, Katie, fired 15 years ago from her job as an analyst at the agency without explanation, now writes a TV show called Spy Guys. She still wonders why she got the boot, but has tried to let it go—until she gets a call from an old co-worker promising information about her dismissal. When the caller disappears, Katie's right back into it, and she embarks on a mission to find this woman and discover what she knows. The search leads her through a trove of memories of her days at the CIA, with the fall of the Berlin Wall (which was happening when she was at the agency) serving as the backdrop for espionage and sleuthing.

To research the novel, Isaacs met with some former CIA agents, though her goal was to learn their methods, not to uncover information. "Look," she says, her New York pedigree obvious in the accent that spices up her speech, "nobody's going to give me secret stuff. Nobody's going to break the code for me. And I don't want them to. What I want is: How does it operate? I want to hear them tell whatever they're going to tell for the language, for their style."

While her characters may come from her experiences, Isaacs is careful to make sure the narrator—not the author—tells the story. If a reader says of one of her characters: " 'Oh, that's Susan Isaacs Long Island Jew/mother/grandmother blah blah blah,' then I've failed," says Isaacs, "because I've brought you back into my world, and I've taken you away from the universe of the character."

Isaacs is just as careful with her writing/editing method. No one sees the novel until it's finished (except for her husband, a criminal defense lawyer). This process began with her first editor, HarperCollins's Larry Ashmead, who edited her books for 20 years before retiring, and continues with Graham. Isaacs moved to Scribner after Ashmead retired, lured by publisher Susan Moldow, with whom she'd worked at Harper. For Graham, the editing arrangement works well: "The danger of not seeing something along the way is that it will have gone astray." But Graham knows Isaacs is a pro.

Just as Isaacs doesn't show her novels to others until they're finished, she doesn't advise new writers to share their work in writing courses. "You're exposing yourself to too much danger," she says. "If you're an original voice, you run the risk of being pushed into something else, or losing it." Isaacs herself learned to write from a book (John Braine's 1974 How to Write a Novel), and believes the solo track works best.

Yet she's a strong supporter of the writing community. She's chairman of the board of directors of Poets & Writers and communes with fellow writers through that organization as well as with a group of writers that includes notables like Mary and Carol Higgins Clark, Harlan Coben and Linda Fairstein. They meet for lunch monthly. It's a free-for-all through the salad course, when the talk turns professional and they share advice about technical writing problems and discuss their agents, editors and what's going on in the business.

As for a reading community, Isaacs's is terrific, but she's still eager for new fans and is pleased to notice interesting additions to her audience at recent signings: younger people, more males and more African-Americans. Moldow says Scribner is "reaching down in age" for Past Perfect with jacket art designed to be contemporary and appeal to readers of all ages. And Graham adds that the house is gathering blurbs from younger writers. Penguin will publish new mass market editions of Any Place I Hang My Hat (Pocket Star) and Compromising Positions (Berkley Trade) in February, which should attract even more new readers.

Once a book is finished, Isaacs happily tours. "Listen," she says, "there's the public writer and the private writer. The public writer has nothing to do with writing. You're an ambulatory ad for your book. If your persona is somewhat in keeping with the kind of writing you have, great. If you write hilarious books and you're a total lead-ass, maybe consider staying home."