MOSCOW (AP) _ Considering she might be Russia's most popular woman, there is nothing smug or flashy about Anastasia Kamenskaya.

She doesn't wear makeup, except when her work demands it. She dresses plainly, with the same exception. She works hard, and she works smart.

``She is a modern young woman,'' observes Liza, a bookseller at Moscow's bustling Kievskaya Metro station. ``She's a little bit reserved, but nice enough. Many women like her appearance.''

Kamenskaya, known to millions as Nastya, is Russia's favorite cop. She is also, alas, a work of fiction, the heroine of the country's best-selling mystery novels.

Mysteries, which were scarcely available in the Soviet Union, have become hugely popular in post-Soviet Russia, where crime _ and the fight against it _ has become a national obsession.

At first, Russians read mainly American and British authors, hastily and poorly translated. Now, however, a whole generation of Russian mystery writers has skulked onto the scene.

None is more successful _ or intriguing _ than Nastya's creator, Lt. Col. Marina Alexeyeva. A full-time law enforcement official in the Interior Ministry who writes with an insider's knowledge of crime and detection, she shares many of the qualities of her fictional alter ego.

Soon, however, she is likely to part paths with Nastya, a poorly paid civil servant, in one significant respect. Alexeyeva is undoubtedly going to be rich.

Five of her books were among the 10 best-selling paperbacks in a recent list compiled by the weekly newspaper Book Review. She has sold nearly 9 million books _ 6 million in the first nine months of this year _ under her pen name, Alexandra Marinina.

Three Russian television networks are said to be negotiating with her for series' rights. And outside Russia, she and her agent are talking to publishing industry people in five countries about rights to translate her books.

``She is definitely Author No. 1,'' says Gennadyi Kuzminov, the editor of Book Review's best-seller list. ``This is a social phenomenon, not a literary one.''

On the eighth floor of a Moscow high-rise, the phenom answers her doorbell in the midst of a coughing fit. Like Nastya, she is a smoker. Despite this, there is something athletic in her bearing.

She is a tall woman with big features and oversized, plastic-framed glasses. She is 40 years old and wears her brown hair tied in a bun. She is dressed plainly, in a black pantsuit. She wears no makeup.

Her apartment is cozy, meticulous and crammed with books, all neatly filed by author. There's a row for Ed McBain, who writes American police novels; for Jackie Collins, Alexeyeva's guilty pleasure; and for her favorite author, Sidney Sheldon.

``He's such a master of plot and suspense,'' she says.

There's also a shelf, right above the computer in the living room, devoted to the works of Marinina: 19 books and counting, all written in a remarkable blaze of ambition since 1991.

Before that, her writing had been confined to official reports _ crime forecasts and the psychology of the criminal mind _ in her capacity as a police researcher. In 1984, she became a manager in the Interior Ministry's law enforcement training academy, a position she still holds.

She wrote her first detective novel ``to see if I could do it.'' She could, and hasn't stopped writing since.

Her timing was perfect. Before the late '80s, the only mysteries available in Russia were those by a few carefully selected Western authors _ Arthur Conan Doyle, Georges Simenon, Agatha Christie.

``I couldn't have written these 20 years ago,'' Alexeyeva says. ``I couldn't have published them here because, not being a member of the Communist Party, I couldn't have been a member of the Writers' Union.

``Besides, I couldn't have written the things I write now _ that not only angels work for the police, but fools and drunks, too.''

She writes all that, and about contract killers and Mafia bosses and official corruption. Russians can't get enough. It is rare to walk onto a subway train in Moscow and not see someone reading a Marinina novel.

When she is asked why she is so popular, her agent, Natan Zablotskis, who is also her boss at the Interior Ministry, pipes in.

``One can find other thrillers with more exciting, bloody or thrilling plots,'' he says. ``Marina writes psychological books, and any person reading her work can see the problems that he or she runs into every day _ love, hate, justice, selfishness, revenge.''

Her books are not loved by critics. ``Exceptionally badly written,'' scoffed one reviewer in the Moscow News. Her success feeds into a perception in critical circles that Russian tastes _ artificially maintained during the Soviet era, when readers tended to choose Russian classics over the unpopular drivel of party hacks _ have plunged into the toilet.

Alexeyeva will have none of this.

``I don't understand this snobbery,'' she says. ``I write about things that make me think and that concern me. And if there are so many people who find them interesting, I am very grateful to them and I think it shows I do my job well.''

When she began writing, she says, her publisher wasn't sure there would be a market for her books. Seventy percent of Russian book buyers are women, he told her, but 70 percent of mystery readers are men.

He suggested she try writing a book about a male detective, using a male pen name. While she was writing it, her first Nastya books began flying off bookstore shelves. Women, it seemed, did like mystery novels when the protagonist was a smart, confident, successful woman.

So did men.

Never mind, her publisher told her _ give us more Nastya. Her ``men's'' book was published _ under her name.

Kuzminov, the best-seller editor, says Alexeyeva's books succeed because ``they meet people's expectations _ the good always wins.''

``People still think they ought to rely on their intellect, and her heroine wins because she has brains to the criminals' brawn,'' he adds. ``Our readers are still romantic enough to believe that intellect can defeat strong-muscled evil.''

So far, Alexeyeva has not made a fortune from her books. For one thing, books are not as profitable in Russia as in the West. A hardcover book sells for only about 18,000 rubles ($3), a paperback for 7,000 rubles (a little over $1).

She is making enough, however, that she is thinking about quitting her day job. Considering that she wrote five books last year, working nights, weekends and vacations, this can only be good news for her fans.

And for her. Sitting at her kitchen table, she is asked if she enjoys writing. She looks sideways out the window for a radiant moment. ``Yes,'' she says softly.

And Nastya? Will she keep writing about her heroine or is she growing tired of her? No, she says, ``I like her. I'm planning to live with her.''

Millions of Russian readers will be envious _ and glad.