Ten years ago, when she left her publishing job to write children's books full time, Ann M. Martin thought she knew what she was getting into. She would publish a couple of books a year, if fate and editors were kind. She would have a peaceful, semi-anonymous life. Such prospects as unlisted phone numbers and security guards at bookstore appearances never entered the picture.

"I don't like being onstage," Martin says. She is slipping off her shoes and tucking her feet beneath her on the sofa; a striped kitten named Woody takes up residence in her lap. "I thought I'd write a book, see it in the bookstores, then go home and write another. This isn't something I would have chosen for myself."

This being the hoopla -- floods of fan mail, promotional tours, merchandising and, soon, the movie -- that comes with being one of the world's best-selling juvenile authors. A decade ago a Scholastic Inc. editor noticed that titles featuring clubs and titles about baby-sitting registered strong sales and suggested that Martin attempt a series of girls' novels about a baby-sitting club. As an idea, this was roughly akin to, say, envisioning that Americans might wear trousers made from riveted denim.

"Who knew what they would become?" recalls Diane Roback, children's book editor of Publishers Weekly. "The industry hadn't seen that kind of phenomenal success with a series before. Or that staying power over many years."

Grade-school girls everywhere now wait impatiently for each month's new volume of "The Baby-Sitters Club," set in fictional Stoneybrook, Conn., where a covey of 13-year-olds weathers interpersonal crises and triumphs over (fairly minor) adversities. Fans of the series are apt to amass dozens of the 200-plus titles, including spinoff series and various special editions. Kristy, Claudia, Stacey et al. are probably the most read-about girls since Nancy Drew and her chums.

The BSC's conquest of children's bestseller lists inspired an even more successful Scholastic series, R.L. Stine's boy-friendly "Goosebumps" books, that has lured away some of Martin's potential readers. The BSC movie, in the works for three years and hitting theaters tomorrow, may have materialized just in time to stanch a sales decline. But Martin's girls, their sales reaching 125 million volumes, remain well ahead of whoever's in third place.

Martin still seems a bit startled by it all. Here in her country house, with an expanse of the rolling Catskills visible from the rocking chairs on the porch, it's easy to imagine the simple existence -- one woman, two cats, novels written longhand on legal pads -- she originally had in mind. But Ann M. Martin isn't just a writer these days; she's an industry. A brand name.

"I'm responsible for 12 Baby-Sitters Club books a year," she says, ticking off her daunting workload. "Twelve Little Sister books {the spinoff for younger girls}. Six mysteries and about four Ms. Colman books {another Scholastic series about to debut} and two or three other titles. . . . It totals over 30 books a year. I don't think even Stephen King could do it."

As a result, Martin no longer actually writes all the books that bear her name, though she does outline and edit them all, tinkering until other writers' manuscripts sound Martinlike. "Kids know I'm not the actual author of each one," she says. This is nowhere acknowledged on the book jackets, but "it's gotten out. They ask how I can write them all and I say, I can't.' "

In addition to a stable of ghostwriters (and translators, since the books appear in 19 languages), the Baby-Sitters biz includes people who handle Martin's mail -- 17,000 letters annually. And people who monitor the Baby-Sitters Club conference on Prodigy, selecting several letters a week for Martin to respond to on-line. And a child psychologist, a consultant to Scholastic, who replies (over Martin's signature, of course) to communications from troubled kids.

There are merchandise specialists busily licensing BSC trading cards, dolls, board games. "I didn't want a lot of makeup stuff," says the author, who rides herd on everything related to her creation. "A makeup kit for 9-year-olds? We settled on bath soaps."

And now there are movie publicists as well, as the Baby-Sitters -- previously seen on cable -- hit the big screen. Martin read every version of the script, talked with director Melanie Mayron, viewed young actresses' audition tapes. She was also supposed to play a cameo role during a brief visit to the set. Now she's glad that didn't happen. "I couldn't do it, it was waayy too much onstage," she says. "I would have been so incredibly self-conscious."

Like many of her young readers, Martin particularly identifies with one of the Baby-Sitters. It would be helpful -- given the film premiere in Los Angeles and an upcoming tour hitting every single state over two years -- if that character were Kristy, the ebullient and slightly self-promoting club founder and president. Alas, Martin feels closest to sweet but introverted Mary Anne. Growing up in Stoneybrookish Princeton, N.J., "I was definitely her personality -- shy, quiet and, especially when I was younger, not very outspoken," she says.

Even now, just turning 40 though her wiry build and delicate features make her look younger, she's stopped giving speeches at schools and teachers' conventions. "I couldn't stand it," she confesses. "I was terrified, I hated it, and it never got easier. I finally said, Why am I doing this?' "

Not that Martin doesn't want to meet her readers; she does, usually at bookstore events. "I've seen kids dressed up as Baby-Sitters Club dolls," she recounts. "I've signed everything from hands to T-shirts. They bring their favorite books that look like they've been read 50 times. I hear them chattering madly in line but they get shy when they meet me. There's a lot of nudging from parents -- Don't you have a question for Ms. Martin?' " Martin, briefly an elementary school teacher before beginning her publishing career, understands.

But no classroom could have prepared Martin for crowds that average 400 kids, and sometimes swell to 1,000. A security detail has become mandatory. "Once a crowd followed me from a talk at a library to the airport, and surrounded me while I was trying to get on a plane," she recalls woefully. This wasn't children's doing; it was parents fighting to get Martin's autograph for their kids. "It makes me uncomfortable," Martin confesses. "I don't think I'm all that interesting. I couldn't see myself following anyone to an airport."

Her phone is unlisted here and in Greenwich Village, where she spends half the week, but she knows that another Ann Martin listed in the Manhattan directory gets frequent phone calls from little girls. When she bought this turn-of-the-century home on a hillside four years ago, "somebody with a mouth told a few kids" and now "every child in the schools knows where I live." Wary of being approached, she asks visitors not to publicize the town where she lives.

Aside from the loss of privacy, though, Ann Martin is having a nice life, thanks. "I like the ideas the books promote," she says. "Friendship, volunteerism, running one's own business, being independent. If kids reading the books get a dose of that, that's great."

Now and then, parents or critics ask how placid Stoneybrook manages to escape random violence, teen pregnancies and drug abuse. In her defense, Martin points out that the suburb isn't Never-Never Land: several of the girls have divorced parents and step-siblings to cope with; Stacey must manage her diabetes; Claudia's beloved grandmother died; Jessi's family, African Americans in this predominantly white series, faced some nasty racism when they moved in. "But I don't think it would be realistic to bring up every problem that exists, not in this one small suburban town," Martin objects. Besides, "somewhere in the back of my mind is the notion that kids as young as 6 are reading these books." She'll allow the death of a classmate, perhaps, but not a parent; she'll refer to fear of crime without subjecting her characters to the real thing. Stoneybrook stays safe. Friends, despite some bumps, stay friends.

In fact, Martin fittingly stays friends with the Princeton girls who shared her childhood and helped inspire her characters. She borrowed her friend Claudia's name for the arty Baby-Sitter with the junk food addiction and the lousy spelling. Now Martin (who hasn't married or had children herself) is godmother to Claudia's three children, whose photos are displayed on her mantle.

And the bouncy Kristy was modeled on Martin's best friend, Beth. "We started a number of clubs and they were all her idea," Martin remembers. "They lasted for about two days, but it was like the old Judy Garland-Mickey Rooney movies: Hey, let's start a club.' We'd meet in Beth's bedroom, eat cookies and then go home."

No wonder Martin writes effectively about girls and friendship. She and Beth started spending Christmas Eves together when Martin was 5. Now that they're all grown up, they still do. "We make gingerbread houses every year with her girls," Martin says. "I don't think we've missed a year." CAPTION: Ann Martin, author of the "Baby-Sitters Club" series for young readers, is responsible for more than 30 books a year. The series has inspired a movie that opens in theaters tomorrow. CAPTION: Author Ann Martin enjoys meeting her young readers: "They bring their favorite books that look like they've been read 50 times," she says.