One of the biggest success stories in U.S. publishing in recent years has been the continued growth of digital book publishing. Last year, total revenue for e-book sales in the United States reached $3.04 billion, a 44.2% increase on 2011’s numbers and a figure all the more impressive when you realize that growth is additive to the print publishing industry. Even more surprising, publishers have focused much of their attention on genres like sci-fi, fantasy, mystery and romance fiction — markets that have traditionally lagged behind “literary fiction” in terms of sales.

In the last few months, however, Random House and HarperCollins launched their first digital-only imprints, and all of them focused on genre fiction. Random House announced the sci-fi/fantasy line Hydra, mystery line Alibi, “new adult”-targeted Flirt and romance-centric Loveswept, while Harper Collins created the digital mystery imprint Witness in April. Although this focus on genre fiction might seem counter-intuitive according to traditional print publishing sales, Random House VP and digital publishing director Allison Dobson says there’s a simple reason for it: The digital audience wants different things.

“Certain categories [of eBooks] have a much larger digital adoption than others,” Dobson said. “The genres were among the first where readers took to the digital format and the ratio of readers of digital, as opposed to physical, are much, much higher.” In the case of some genre titles, as much as 60 to 70 percent of the sales are digital. “I think there is an enormous audience in digital right now,” Dobson said. “It’s actually where the action is.”

There are multiple theories for the genre dominance in digital publishing, including the appeal of anonymity offered by e-reader devices, which don’t display the cover of a potentially embarrassing book for all the world to see. As Antonia Senior wrote in The Guardian last year, “I’m happier reading [historical romance fiction] on an e-reader, and keeping shelf space for books that proclaim my cleverness.”

But the digital delivery system also offers immediacy and ease of access for material that often is serialized and written to make you want to know what happens next, as soon as possible. Liate Stehlik, senior vice president and publisher at HarperCollins, subscribes to that idea, at least partially. Genre fans, she says, became “early adopters” of the digital format because e-books are the optimal format “for people who want to read a lot of books, quickly and frequently. Digital has replaced the paperback, certainly the paperback originals. I think the audience that gravitated to eBooks first really was that voracious reader, reading for entertainment, reading multiple books in a month across multiple genres.”

For both Random House and HarperCollins, moving to a digital-first publishing model not only offers a higher return on investment for genre publishing, but also opens the door for those publishers to experiment in a much more cost-effective way than print. “It’s not that we couldn’t publish these books before,” Dobson said, “but [now] that a certain consumer has migrated online, and the ease of buying these books has grown that consumer base substantially.”

“The thing with digital is that you’re not as adhered to a single format or price point as you were in the past,” said Stehlik. “You can do a novella, you can do a short book that leads into a longer book, or a book that bridges two different books from the same author. Before, you might have thought, Oh, there’s nowhere to put that, we’ll have to put in paperback with the next one, but digital presents a different market to promote shorter works. And the audience responds… We don’t have to feel limited by format in the way that we may have done before.”

Digital publishing also allows books to go to market much more quickly than printed books, and offers publishers the benefit of both rapid consumer feedback and the ability to adapt to reader response. “Before, you had to wait, you had to put the books out there, wait six months to see what came back and you’d have to think, ‘Well, maybe if it had a different cover it would work, maybe if it had a different title,'” said Stehlik. “Now, it’s a lot more instantaneous, and you can change the cover, change the title, and see how people respond. You can even engage the audience before you publish, which gives them a kind of ownership over the book.”

But if the digital market opens up new opportunities and options for mainstream publishers, it also puts them in competition with smaller and self-publishers in the digital market. According to a recent survey, more than a fifth of all genre e-books sold in the United Kingdom are self-published, and the phenomenal success of Fifty Shades of Grey (originally self-published digitally by author E.L. James) has handily demonstrated that digital self-publishing doesn’t necessarily bear the same print stigma of “vanity press.”

Both Random House and HarperCollins hope their digital imprints will appeal to new writers and those who have self-published digitally in the past, particularly since it can lead to print publication as well. “With the new authors we’ve worked with this far, at least half of the titles, we’ve been able to sell print editions of those books as well, some in some of the bigger chains such as a Target or a Walmart,” said Stehlik. “For us, it’s the first time in about fifteen years that we’ve actually increased the number of authors and books that we’re publishing. It’s a great opportunity for us to grow our list, and our reach.”

Both Dobson and Stehlik says that they’re excited by the potential of digital publishing, and the tools it gives authors and publishers to experiment with new content and interacting with their readers in different ways. “It’s hard to say that books have one single monolithic future, or path to the future,” said Dobson. “That’s thrilling for people who love this business, because there are lots of ways to take it depending on what kind of content you’re involved in.”

“There’s so much potential,” agreed Stehlik. “It’s very freeing to know who your customers are, and it’s very exciting to have that ability. We’re very lucky to have that today.”