Bridget Collins was about to quit writing when she came up with the book that would become The Binding, a lush historical fantasy above love, loss and desire in a world where books are used to “bind” memories ensuring that bad or troubling moments can be forgotten.

Collins, who had begun her career in young adult fiction, was between contracts. She wrote her first adult novel because it was “the book I wanted to read”. It has already sold more than 35,000 hardback copies – and 100,000 when eBooks are included – making it the biggest-selling fiction hardback of the year so far, an achievement driven largely by the support of booksellers and word-of-mouth recommendations.

“I didn’t think about whether a teenager or an adult would read it or what market I was writing it for,” she says. “But I also really wanted the story to be immersive, almost overwhelming and escapist in the way that the best young adult fiction is while at the same time hopefully doing interesting literary things.”

Collins is not alone in thinking this way. While there has always been movement between YA and adult fiction with bestsellers such as Angie Thomas read by both adults and teens, writers from Sarra Manning to Daniel José Older carving out successful careers in both genres and everyone from Carl Hiaasen to Nick Hornby giving YA a whirl, the boundaries between the two markets are collapsing and a number of big-hitting YA writers will bring out their first adult novels this year.

“I think there’s always been a blurring of YA and adult fiction but people weren’t as aware of it,” says Irish author Louise O’Neill, whose first two novels, Only Ever Yours and Asking For It, were sold in both YA and adult fiction. “Books like The Outsiders, Catcher in the Rye or Curtis Sittenfeld’s Prep could just as easily be placed on the YA shelves in bookshops today. When I was writing Only Ever Yours I didn’t set out to write a book for young adults. I just told the story the way that I felt it needed to be told – that did cause some difficulties because many publishers struggled to see how they would market it. Was it YA? Was it adult fiction?”

That blurring is particularly pronounced where fantasy is concerned, says Rachel Winterbottom, senior editor at Gollancz who this year will bring out YA juggernaut Leigh Bardugo’s first adult novel Ninth House. “There’s definitely a move going on which you can see with Leigh, Victoria Schwab – who is Victoria for the YA books and VE Schwab for the adult ones – and also Sarah J Maas, who is writing her first adult fantasy series,” she says. “Part of it is that the adult genre does allow them to go a bit darker and explore other themes. YA itself does cover a lot of dark themes but branding it as an adult book is a way of announcing very early to the audience that this is something different. I think also it allows writers to keep their readership even as they perhaps age out of young adult books. With Leigh’s book, even though it’s very different from her YA fantasy because it’s set in our world, albeit with supernatural elements, it’s still recognisably her voice and her addictive way of telling a story. That’s a big part of the appeal.”

YA juggernaut Leigh Bardugo.

And, while some of this change is driven by market forces – YA fiction recently suffered its lowest sales in over a decade – many authors find their focus and interests shifting over time. “With my last couple of YA books, Hidden Among Us and The Hidden Princess, I realised that my focus was increasingly with the older characters,” says the Carnegie-medal nominated Katy Moran., whose new novel Wicked By Design is the second in a series of adult historical fantasies set in a world where Napoleon won the battle of Waterloo. “I also think that while there’s no theme you can’t cover in YA or children’s fiction you do have a responsibility to your reader when writing for teenagers which you don’t have when writing for adults. So for example in False Lights [her first adult novel] Crow, the book’s hero, would have had to be the villain because of how he behaves towards his younger brother Kitto. That’s not to say that you can’t be complex or nuanced when writing for teens because the very best YA is exactly that, more that you just need to keep your audience in mind.”

There are signs that that YA audience, which is largely progressive, outward looking and interested in issues of representation, may have a long-lasting impact on adult fiction. “I’m always struck when I do school visits by how progressive the students are,” says O’Neill. “They’re incredibly eloquent, able to articulate issues of race, gender and sexuality in a clear and concise way and they expect the books they read to match their progressive outlook. YA authors are expected to be well-versed in these issues, which is not a bad thing despite what some would have you believe and I’m interested to see if that will begin to seep into adult fiction because I’d love to see authors for any age category feel encouraged to take risks with their work.”

For Collins, whose next book due out in 2020 follows a group of students in an alternate Europe in the 1930s, that shift is already occurring. “I definitely think that The Binding would never have been published ten years ago,” she says. “There was a snobbery about what constituted serious fiction and what kind of story we had to be telling. The success of the YA market over the past decade has challenged those expectations and The Binding definitely benefited from that.”