(CNN) -- It's a formula that's been tried and tested in countless erotic novels in recent memory.
A rich, handsome man charms a young innocent with lavish gifts and bold declarations of desire. With a smoldering gaze, he melts her insides, turns her legs to jelly and paralyzes her subconscious, paving the way for hot sex scenes while she attempts to crack his steely veneer.
Formulaic as it is, E.L. James' "Fifty Shades of Grey" trilogy is poised to become one of the fastest-selling series in recent years, with 20 million copies sold in the United States and 31 million worldwide since March. Movie rights are spoken for, and in addition to becoming the subject of online parodies, a "Saturday Night Live" sketch, a musical and hotel marketing gimmicks, the S&M-flavored love story of a recent college grad and a billionaire CEO is also credited with boosting sales of sex toys, driving women to hook-up sites and fueling a craze over sexual domination.
What's behind the trilogy's runaway success? The books hit the romance novel scene at just the right moment, insiders say, riding a wave of smart marketing and benefiting from the erotica world's e-book savvy. Plus, if the plot and characters sounds a bit like "Twilight" fan fiction, it's because they're modeled after them.
The most potent (and obvious) factor is that sex sells, and kinky romances like "Fifty Shades" are sating women's lust for erotica-lite, said Jaclyn Friedman, author of "What You Really Really Want: The Smart Girl's Shame-Free Guide to Sex & Safety."
"The popularity of 'Fifty Shades' speaks to the fact that the dominant model of mainstream pornography and ideas of sex are targeted at men. The fantasy starts when he's into it and ends when he's done," she said.
"A lot of women have said that they've learned the most about what sex could be for them from erotica novels," she said. "These stories focus on female desire and what's in it for the woman, and there's not a lot of that in mainstream culture."
Whatever the case, sex has long been selling erotic tales, earning spots on best-seller lists and testing attitudes toward sexuality. The heroine in French author Pauline Reage's "Story of O" consented to being whipped and branded more than 50 years before Anastasia Steele signed Christian Grey's non-disclosure agreement and terms for becoming his submissive in "Fifty Shades of Grey."
In the interim, dozens of erotic romance novels by authors such as Eden Bradley, Sylvia Day and Megan Hart have landed on best-seller lists, firmly establishing the genre as a major player in the publishing industry. Romance readers, known for their voracious literary appetites, were among the first adopters of e-books, said Michelle Renaud, spokeswoman for publisher Harlequin, which has been carrying erotic novels since 2006.
"They love to consume many novels at once, and e-books are an easy way to carry them around."
The "Fifty Shades" books were originally released as e-books by the Writers Coffee Shop, an independent publisher based in Australia. They were instant hits, drawing attention of fans and Vintage Books, which acquired the rights in March and published them as e-books first and then as paperbacks.
" 'Fifty Shades' is no different from what's already being sold. The major difference is that it has somehow become a part of current pop culture, of the zeitgeist. People won't stop talking about it, so it perpetuates the sales, perpetuates the mythos, of this work as some sort of watershed for erotic fiction," said Mala Bhattacharjee, features editor of RT Book Reviews, the pioneering literary review journal formerly known as Romantic Times.
"In truth, romance fiction and erotica have been this 'naughty' for decades — naughtier, even!" she said in an e-mail. "In essence, it's the phenomenon of FSOG that has snowballed its popularity for readers more than the actual text."
If so, much of the success of the "Fifty Shades" trilogy may simply reside in its packaging. After acquiring English rights in March, Vintage Books released digital editions of all three books, followed by print versions in April featuring relatively tame covers compared with bodice-rippers of yore. Where one might expect two milky bodies intertwined, the covers of books one and two feature a gray silk tie and a bedazzled face mask.
"We felt that discreet, tasteful covers certainly would being in new readers who may not have bought the books with more explicit covers," Vintage Books spokesman Russell Perreault said.
The formula seems to have paid off, and since then, the romance industry has been keen to reap the benefits of judging a book by its cover. Not long after the series took off, Harlequin and other publishers began reissuing old titles with toned-down covers.
"What may have tipped the scale for the 'Fifty Shades' trilogy in particular are the nondescript covers. For whatever reason, the classic 'clinch' covers on a lot of romance novels tend to carry a stigma of being 'old-fashioned,' so the covers on 'Fifty Shades' may have made the books more approachable for a larger range or readers," RT Book Reviews Editor Audrey Goodson said.
"Certainly, there are some who might prefer to read romance on the privacy of an e-reader, but what's really refreshing for me to see is the large number of women reading 'Fifty Shades' on the subway and at the coffee shop. It simply proves that while covers may play some role in readers' preferences, it's what's between the covers that really counts."
Updating a cover isn't new either, but using "iconic" or "object" imagery picked up speed a few months back in an effort to capture the momentum of "Fifty Shades."
"It sends a visible signal that if you liked 'Fifty Shades,' there are other titles you'll like," said Monique Patterson, executive editor with St. Martin's Press, which is reissuing titles by best-selling authors Lora Leigh, Laura Reese and Opal Carew with new covers this summer. "Tastes tend to evolve and change over time with any product, not just books. As time goes on you, have to refresh and update your way of speaking to your audience."
Best-selling author Sylvia Day's first self-published title, "Bared to You," entered the New York Times best-seller list in April despite its cover, which featured a profile shot of a naked woman with her arms wrapped across her breasts and a man on his knees, his face pressed into the small of her back. When Berkley Books took over the title in May, the cover art changed to an image of a pair of cufflinks. The title is No. 4 on the New York Times best-seller list, right behind the three "Fifty Shades" books, and can be found on bookshelves in Target next to "Fifty Shades of Grey."
"Many new readers have discovered erotic romance and want to read more, but the more explicit covers don't appeal to them," said Cindy Hwang, executive editor of Berkley Books. "Publishers are smartly responding to the marketplace and adjusting the cover approach for some erotic romance titles. Cufflinks and keychains can be just as evocative as bare skin."
Day says she has received mixed reaction to the change from fans. Those who self-identify as avid readers and rely on cover art to identify the myriad subgenres within the romance category complained that the new cover obscured the novel's meaning. Others, however, welcomed the change and said it led them to finally purchase the book, where before, they didn't want to be seen carrying it around.
" 'Fifty Shades' readers and new readers of 'Bared to You' don't tend to self-identify as readers per se, and they want books they can take out in public, whereas romance readers don't care. They have adopted strategies over years, with e-books and book covers, and they'll go out with a book regardless of the cover," she said. "For those who are getting back into reading with 'Fifty Shades' and 'Bared to You,' it's a very new thing they're stepping into, and they want it to be an easy transition."
E-books have also helped to ease the transition by offering newcomers to the genre a way to read in virtual anonymity, said Twanna A. Hines, a New York-based writer, sex educator and reproductive rights advocate. Its second wave of popularity came through word of mouth once it started receiving widespread coverage in mainstream news outlets and popular culture.
"It's been parodied on 'SNL' and on the cover of Newsweek. Who wants to be left out of the conversation?" she said. "Plus, BDSM is appealing because while many Americans have heard of 'Fifty Shades of Grey' and can tell you it's an erotic book, not as many can tell you what the letters BDSM stand for, so there's this allure, the siren call to find out more."
For the next few months, expect publishers to bet on that desire and reissue more BDSM titles from the past dressed up in scarves, leather belts and feathers until the next trend rolls around.
"You can't create a moment like this, because if you could, we'd do it all the time," said Patterson, of St. Martin's Press. "It just ends up that the stars align and something breaks in a major way at the right time that ends up hitting readers and grows like wildfire. You can't really explain it all the way. It's just one of those moments."