“Could you murder your wife to save your daughter?” That’s the hook for a novel that has enabled self-published author Adam Croft, writing from his back bedroom, to pay off his mortgage in just 20 weeks, selling 150,000 copies and winning a book deal with Amazon.

Croft, who lives in Flitwick, Bedfordshire, was running an internet marketing company when he wrote his first novel, Too Close for Comfort, in 2011. At first, he wasn’t sure what to do with the thriller, in which new recruit DS Wendy Knight takes on a sadistic serial killer. “It didn’t cross my mind to send it to a publisher – I thought it was my first one and it wouldn’t get picked up,” says the 29-year-old. He looked into Amazon’s self-publishing programme Kindle Direct Publishing, which offers royalties of up to 70%, depending on pricing. “I just wanted someone to pick it up and read it. I didn’t know I could self-publish until two weeks before I did it.”

Sales “trickled” in at first, until they took off enough for Croft to decide that he would write full-time. Croft has since self-published eight more books, seven of which are parts of his Kempston Hardwick and Knight and Culverhouse series. Amazon does not release sales figures, but Croft says he had sold around 350,000 books in five years, until the gamechanger: his most recent novel, Her Last Tomorrow. This thriller has sold 150,000 copies in just five months, and Croft estimates that he’s on target for £1m ($1.4m) of sales in 2016, compared with £20,000 ($29,000) in 2015. To put this in context, a 2014 survey of almost 2,500 working writers in the UK found that the professional author’s median income was just £11,000 ($16,000), while a 2015 survey of US authors found the median fell below the poverty line – a plummet attributed to “several factors related directly to Amazon’s role in the current publishing landscape”.

Publishers have had it very easy for a long time – since presses were introduced, there’s been very little change Adam Croft

“It went a bit mad to say the least,” says Croft of his last book. “I went from just about paying the bills to making around £2,000 ($2,900) a day in royalties. It got into the top 10 in the Amazon digital chart and last week was No 12 on the overall Amazon paperback chart, fiction and non-fiction combined. We’ve paid the mortgage off just 20 weeks after not having a pot to do the proverbial in. It’s totally blown everything through the roof in the space of a few weeks.”

Croft’s success comes in the wake of a new report from Enders Analysis, published by the Bookseller, which found that 40 of the 100 top-selling ebooks on Amazon US in March were self-published. Self-publishing is “only going to grow more attractive” as an option for writers, said the report, which went on to warn that it was “the largest threat to incumbent publisher businesses in the medium term ... and publishers cannot be complacent”. However, the medium has not won over everyone: founder of Profile Books Andrew Franklin said an “overwhelming majority” of self-published books “are terrible – unutterable rubbish” while author Jonathan Franzen said Amazon’s self-publishing model favoured “yakkers and tweeters and braggers”.

“I’m happy enough for the traditional publishers to be sniffy about self-publishing – they’ve been sticking their heads in the sand for years and it’s to their detriment,” says Croft. “Publishers have had it very easy for a long time – since presses were introduced, there’s been very little change. For years we’ve been saying that this is the way things are going, that they’re changing. We’re not saying this for our own benefit, because we’re already embracing the change.”

The author has signed a deal with Amazon’s crime imprint Thomas & Mercer, to republish Her Last Tomorrow and release his next novel, provisionally titled Only the Truth, in 2017. “We’re delighted that Adam has chosen to join the Thomas & Mercer imprint – we’re looking forward to helping bring his pacy, dark thrillers to a wider audience,” said Eoin Purcell, UK leader of Amazon Publishing. “Adam is an exciting voice in one of the most interesting genres in crime writing right now, whose work really explores the psychological depths of his characters. We’re excited to help introduce him to new readers.”

The paperback of Her Last Tomorrow is currently 29th in Amazon.co.uk’s bestseller list, just behind A Game of Thrones. It has almost 1,300 reviews from readers, around 1,000 of those five-star, and Croft says it’s been the biggest-selling independent book this year so far. “Little old me, doing it myself from my back bedroom, has outsold people like Stephen King and Lee Child over quite a few weeks this year.”