In North America alone, some 50 million women are letting long, lean strangers into their bedrooms. And more women are being seduced every day.

But these romantic visitors are the kind who keep their jackets on. Because though you might not be seeing as many heaving bosoms on the book covers, romance novels are proving themselves the backbone of the publishing industry.

"Although the industry may be shrinking in general fiction, romance is pretty much stronger than ever," said Carol Stacy, publisher of Romantic Times Book Club magazine.

And it's not just pillow talk. Romance novels account for half of all paperback books sold, and bring in an estimated $1.5 billion a year. About 200 titles are pumped out every month, and some of them inevitably nestle into the top 10 best-selling fiction lists. Stacy estimated the average subscriber to her magazine reads between 10 and 40 romances a month.

Some say romance is growing more popular partly because those alabaster breasts are getting covered up, and because the entire genre has undergone major changes that don't necessarily involve feisty Spanish princesses or handsome swashbucklers.

The modern incarnation of the genre started in 1972 when Avon Books published The Flame and the Flower, the 18th century tale of a beautiful orphan girl who is abducted by a dashing sea captain. Several heated whispers, tender caresses and pounding heartbeats later, the two end up happily together.

And if the relationship between the "sapphire-eyed" beauty and her hulking captain was hot, it was nothing compared to the love affair that began between publishing houses and the quickly written novels.

For most of two decades, publishers demanded writers stick to the "bodice ripper" formula: historical fiction with a monogamous hero and heroine, an instant attraction, a "what-the-hell-are-they-fighting-about" conflict and a happy ending. And, naturally, the infamous covers: a bare-chested man embracing a half-clad woman.

But over the last 10 years, the romance genre has changed dramatically, most obviously with those covers.

"It's not been bodice rippers for a long time," Harlequin Enterprises Ltd. vice president Catherine Orr said. "On covers, dresses tended to be extremely low cut, the hero was usually grabbing, nuzzling her. It was overly romantic and sexy for some, and people didn't want to be seen reading that on the bus or the train."

So bare chests moved over for pictures of houses or castles or even plain stylized covers with just the author's name and the title.

More significantly, publishers gave the old formula the heave-ho. When a handful of writers found success with contemporary themes in the 1990s, pirates and nobleman made way for private investigators, neurosurgeons and firefighters.

"Now there are single dads, and they use condoms and they eat low-fat foods," Stacy said. "There's abortion, there's Alzheimer's -- anything happening today. I think the author is recognizing that the reader wants to be able to relate."

The only thing to stay the same is the guaranteed happy ending, she said.

Romance is so healthy that it has borne thriving sub-genres. Very popular are "romantic suspense" novels. There are veritable libraries dedicated to affairs involving Navy SEALs, publishing lines for black, Asian or Latino characters, "Chick Lit" books along the lines of Bridget Jones's Diary, paranormal books in the vein of The X-Files and online romance series.

Last year, there were even plans for more than one book about romances with terrorists, though it's yet to be seen whether they'll make an appearance after Sept. 11's events.

More successful romance writers like Nora Roberts, Janet Dailey, Janet Evanovich, Jennifer Crusie and Iris Johansen have crossed genres, putting out best-selling mysteries and popular fiction. It's gotten to the point that even publishing insiders are hard pressed to define the boundary between romance novels and women's fiction.

But despite the popularity and new face of romance, plus plaudits from celebrities like Kelly Ripa, some say the genre will never get enough respect.

"We're never going to lose the stigma of the bodice ripper," Stacy said.