Bestselling ebooks by authors including Jeffrey Archer and James Herbert are being sold for just 20p from Sony and Amazon's digital stores, prompting concerns from writers that the "relentless downward pressure on book prices" could lead to industry ruin.

As news emerged this morning that digital fiction book sales were up 188% by value in the first half of this year, according to the Publishers Association, Sony added another title – Herbert's bestselling horror novel Ash, which has a cover price of £18.99 – to its 20p price promotion, which has been running all summer. Sony's 20p offer, which has seen Amazon.co.uk price-matching each discount, currently includes a range of titles from Kate Grenville's Sarah Thornhill to Chris Pavone's The Expats.

Readers have been quick to take advantage of the cheap titles: Ash currently sits at the top of both Sony and Amazon's digital bestseller charts, with Alice Peterson's 20p title Monday to Friday Man in third place on Amazon, David Baldacci's 20p Zero Day in fifth, Peter James's thriller Dead Man's Grip in seventh and The Expats in ninth place, all discounted by 97% or more.

James has seen his sales soar as a result of the promotion – "at one point I was selling 10-20,000 copies a week," he said – and as both Sony and Amazon are taking the hit on the offer, he and his publisher are still getting paid as if the books were selling for full price.

"I'm still getting royalties as if it were full price … so I'm a really happy bunny," said James.

But the author feels that while the offer is attractive in the short term, "it has a lot of long term dangers". "What's worrying is that the 20p price point sets a precedent. The public starts getting used to paying even less," he said. And unable to compete on price, the sector of the market that will "lose out in the long term is the independent bookshop," believes the novelist. "That's my biggest worry."

Pointing as well to the "price war" currently under way over JK Rowling's soon-to-be-published adult novel The Casual Vacancy, Kate Pool at the Society of Authors said that "some writers favour cheap books, believing that low prices are made up for in substantially increased sales", but that the majority "find the relentless downward pressure on book prices deeply worrying". Rowling's £20 novel, out on 27 September, is on offer for £9.86 from Amazon and Tesco, and for £10 from Waterstones.com.

"Since before the inception of the Society 128 year ago, authors have fought long and hard to ensure that copyright properly protects their fundamental right to control and profit from their creations," Pool said. "If books are perceived to have almost no value, that fight seems pyrrhic indeed, as are the chances of professional authors, of even the most sought-after books, let alone those which are highly researched or costly to produce, making a living from their writing."

Historical novelist Lindsey Davis, chair of the Society, said that while she approved of special offers as loss leaders, she was otherwise "very opposed to the 20p book trend", because "authors should be able to make a living".

Peter Shea, general manager for Sony Digital Reading Services, said the retailer recognised "that there is a concern about a perceived devaluation of ebooks", and that it chose the price point of 20p for some titles "as we see this as such a significant discount off list price that consumers can appreciate it is not the 'new price of ebooks'."

"As a retailer dedicated to ebooks only, we at Reader Store believe that offering exceptional prices on ebooks that people want to read is an engaging way to help curious readers experience digital books, perhaps for the first time," said Shea. "Unlike with physical books, some publishers do not allow their ebooks to be sold at discounted prices. As a result, at Reader Store we aim to provide excellent prices to our customers where we can with a selection of ebooks priced as low as 20p while others are only available at full list price. Of course, the publishers and authors of the ebooks we choose to sell for 20p still receive their full payment for each book we sell. We have found these publishers and authors enthusiastic when informing them of our pricing for their books."