It seems like yesterday that women's erotica was something to be read in private under the sheets. But no more.

With ebooks like Kindle and Nook enabling discreet downloading and reading, erotic novels for women are swiftly gaining in popularity.

British author E.L. James' "Fifty Shades of Grey" trilogy has been parked atop the New York Times digital bestseller list for weeks, but it's not alone in the genre.

"Sales have always been good," said Tina Haveman, founder of eXtasy books, "but the increase has happened over the last few years because of the new ebook readers that have become available."

"Ebooks are cheaper than paperbacks, and easier to buy because of the convenience of the Internet," she added, "and people can read them without having to hide them from prying eyes."

"You could be a mom, like, sitting in the park on a playday with the moms down the block and you could be reading, like, a real kinky novel and nobody knows," agreed Brenda Knight, associate publisher of Cleis Press.

Founded in 1980, Cleis Press bills itself as the largest queer publisher in the United States, and 70 percent of its erotica sales are digital.

Kensington Books, another US publisher, says its Aphrodisia collection of adult ebooks accounted for 57 percent of sales in 2011, compared to a mere two percent three years earlier.

But its spokeswoman Vida Engstrand said the success of such "mommy porn" as "Fifty Shades" is not singularly responsible for the growth in ebook sales.

"Sales in ebooks have been growing in leaps and bounds since well before E.L. James hit the mark," she said.

"I will say that 'Fifty Shades of Grey' has put erotica in mainstream media, mainstream stores and the public eye," she added.

"My hope is that this sudden cultural awareness of erotica will lead readers and media outlets to take it more seriously as a genre."

It's a big sector.

The Romance Writers of America says that sales of all romance novels with "varying levels of sensuality ranging from sweet to extremely hot" was worth $1.4 billion last year, with 8,240 titles published in 2010, or 13 percent of all books.

One in three titles is bought in digital form, a bigger proportion than for any other genre, the trade group said.

Adam Nevill, who oversees Mischief, a new erotic imprint from HarperCollins in Britain, linked the buzz surrounding "50 Shades" to the previously established popularity of "horror tropes" by the likes of Laurell K. Hamilton, Charlene Harris and Stephanie Meyer of "Twilight" fame.

"There was already a big readership, but this one crossed over into a general readership from a large genre readership," he said.

"The interest in this genre goes back to Anne Rice, too, and all the way back to 'Bram Stoker's Dracula,' too. I think it just evolved into a bigger readership online."

Sarah Wendell, founder of the romance literature blog Smart Bitches and Trashy Books, noted how downloading an ebook can be a welcome alternative to risking unwelcome comments at a bookstore's checkout counter.

"Many romance readers have reported snide comments from booksellers as they brought romance novels to the checkout desk -- even if they weren't erotic romances at all," she said.

"Women who gravitate towards erotic romantic fiction do so for a combination of reasons," Wendell added.

"It's a positive portrayal of female sexuality, where the woman is celebrated instead of denigrated, and it explores different sexual practices within the safety and privacy of the reader's own imagination."

She added: "If you're curious about something you've never tried before, reading about it in a book and imagining it for yourself can be tremendously powerful and educational -- and there aren't many places where women can explore their own sexuality in a safe and non-judgmental manner.

"Plus, some erotic romances are outstanding stories told by very talented writers and are a pleasure to read."

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(c) 2012 AFP