A bestselling debut novelist who wrote her book in a hospital car park as stress release from her job as a psychiatrist is to return to the NHS. Her decision comes despite a £300,000 deal for her second book and a contract for two more novels.

Joanna Cannon, whose first book The Trouble With Goats and Sheep has now sold more than 100,000 copies in paperback in the UK and has been optioned for film by the makers of the Amy Winehouse documentary Amy, said she was returning to the health service because she missed her patients.

“I am hoping to go back in a voluntary capacity, helping patients understand their own narrative,” she said. Cannon will work with Arts for Health, a programme run by South Staffordshire Healthcare Foundation, which brings the arts into hospital for patients.

“I made the choice to do this over the last few months after talking with consultants,” the author added. “The most valuable time I had working in wards was spent with patients, and because I found it difficult to work in psychiatry part-time, this is the perfect solution.”

Cannon’s decision emerged as it was announced that she had signed a deal for two further books with her publisher, the Borough Press, part of HarperCollins. Although she said it was “too early” to discuss books three and four, she revealed that her second would be called Three Things About Elsie. Published in January 2018, it will be about growing old and how elderly people are treated in society.

The author began her first novel after finding work on the wards stressful. Having left school at 15 with one GCSE, she went to university in her 30s and qualified as a doctor in her 40s. Inspiration for The Trouble With Goats and Sheep came from the way she saw society treat patients, as well as the case of Christopher Jefferies, a Bristol landlord who was hounded by the press and falsely implicated in the murder of his tenant Joanna Yeates in 2010.

“Working in psychiatry you meet a lot of patients who live at the edge of society, who are not listened to or even noticed unless something goes wrong,” the author said. “There is that prejudice about how people appear, and I wanted to say we are all a little bit different; it’s just that most of us are very good at hiding it.”

The writer, who lives in the Peak District, became a sensation in 2016 after she entered an X Factor-style writing competition at York festival that led to offers of representation by seven agents and a lucrative book deal for her debut.

The decision to divide her time between writing and working in the NHS was motivated, she said, by her love of the service and because she felt it would provide a balance to the “insularity” of the book world. “Anyone who writes needs to get out there and hear real voices,” she added.

While dealing with her newfound literary fame, Cannon admitted she had found it “distressing” to be away from the wards. “Someone said to me at an event that I used to write to relieve the stress of work,” she added. “Now that writing books brings its own stress, I need to go back to the wards.”

• This article was amended on 30 March 2017, to better reflect Cannon’s position on working part-time in psychiatry.