Talk about stamina: E.L. James‘s “Fifty Shades of Grey” has now been on The Washington Post bestseller list for 50 weeks.

The novel and the other two books in James’s BDSM trilogy have now sold more than 70 million copies world wide. “Holy Cow,” indeed!

And this surely won’t be the climax of her success. With Universal Pictures whipping up a movie version of “Fifty Shades,” sales of the books could continue to surge for months. That’s the kind of domination of the marketplace that her publisher, Vintage, is eager to stimulate. On May 1, James is set to release a “bonded-leather” journal titled “Inner Goddess” for aspiring writers to record “their innermost thoughts.” The first printing will be 125,000 copies.

So much revenue was generated by her three novels that Random House, Vintage’s parent company, was able to give every employee a $5,000 bonus at the end of 2012, hardly the norm in today’s flaccid industry. No wonder James was chosen as the Publishing Person of the Year by Publishers Weekly.

But this British TV executive, who had been writing on-line “Twilight” fan-fiction before “Fifty Shades” launched a thousand op-eds about mommy porn, didn’t invent sex or erotic romance. What is it about her books that made readers so hot and heavy?

Anne Messitte, the publisher of Vintage/Achor who discovered James, says the success of the books is a classic example of the role that word-of-mouth has always played in the success of commercial fiction. “For many readers, these books tapped into fantasy and curiosity, and those aspects started a lot of women talking to other women,” she says. The buzz started among women in their 30s and 40s in the New York metro area but kept spreading. Soon people wanted to know what “everybody” was talking about. “All of that was accelerated by social media and by the general media attention about what was happening and who was reading these books,” Messitte adds. “It wasn’t just a genre-interested reader. It wasn’t just an erotica genre reader. It was a very mainstream, general audience. And that become a story unto itself until it became part of the popular culture very quickly.”

If anybody should be slapping themselves over James’s success, it’s Harlequin. Why didn’t the world’s leading publisher of romance fiction launch this erotica revolution itself?

“Oh, if we had a dollar for every time we asked that!” laughs Susan Swinwood, Harlequin’s executive editor.

Harlequin has many authors tied up writing erotic novels, “frankly, better,” Swinwood says. “It’s just one of those things. It’s lightning in a bottle. If I’d seen that manuscript come across my desk, I don’t know that I would have bought it. But for whatever reason, it hit the market at the right time with the right buzz and the right marketing.”

But Swinwood was quick to acknowledge that “Fifty Shades of Grey” has been a “game changer” for Harlequin. “Erotica had been something you wanted to hide, and women were afraid to be caught reading it. ‘Fifty Shades’ changed all that. Now, when everyone’s talking about it, you feel like you’ve got to find out what’s going on.”

As the popularity of James’s books soared, Harlequin was able to reissue novels from its backlist and find new readers. Megan Hart, a Harlequin author since 2009, saw one of her reissued books hit the New York Times bestseller list after James supercharged the market for erotica.

But not everyone agrees that “Fifty Shades” spread the love far and wide. Sylvia Day, the president of the Romance Writers Association and the author of 19 novels, says, “It’s changed the market in a lot of ways, but I’m not sure it has impacted erotic fiction. We haven’t seen the sort of surge you might expect.” In a sense, James’s millions of readers have remained handcuffed to her books alone.

Day thinks “Fifty Shades” is another peak in the rise and fall of erotic fiction that’s happened before. “In 2006 we saw the launch of erotic imprints from Kensington and Harlequin and HarperCollins. They flooded the market for about a year, and then it disappeared.”

“Fifty Shades” has not spawned “a huge wave of erotic bestsellers,” Day says from her home in San Deigo. “It sells well for particular writers, and they’re mostly the same writers who put their books out in 2006.”

Instead, Day says, the most significant effect of “Fifty Shades” may be on how publishers find new manuscripts and design book covers. Self-published on-line books have become the new slush pile, the place where publishers look for undiscovered gems. And the elegant if innocuous covers of the “Fifty Shades” books have become the new standard even for the spiciest fare. Now, you don’t have to be ashamed to read BDSM “at the soccer game,” Day says.

In the end, “Fifty Shades” may have done more for the past than for the future. Mark Laframboise, the manager of Politics & Prose Bookstore in Washington, notes that the popularity of James’s novels has brought new attention to older erotic works such as Anais Nin’s “Little Birds,” Henry Miller’s “Tropic of Cancer” and  James Salter’s “A Sport and a Pastime.”

And the breadth of James’s audience is extraordinary. “Mostly women, of course,” Laframboise says, “but from 18 to 80.” You’re never too old, after all, to find romance.