Pen and notebook in hand in Edinburgh cafes, Harry Potter author J.K. Rowling has been spotted working on her new book: a detective novel.

Rowling, who famously wrote initial drafts of her boy wizard story in the Scottish city's coffee houses, is turning her hand to crime fiction, the Sunday Times newspaper reported.

Ian Rankin, a fellow author and neighbour of Rowling, told a reporter at an Edinburgh literary festival the writer had recently been seen at work in local cafes, it was reported.

"My wife spotted her writing her Edinburgh criminal detective novel," the newspaper quoted Rankin as saying.

Rankin, famous for his own police novels set in the historic Scottish city, said he had not discussed the project with Rowling personally, and declined to reveal how he had learned about her plans.

"It is great that she has not abandoned writing or Edinburgh cafes," Rankin said.

When Rowling created Harry Potter, she was a struggling single mother who wrote in cafes to save on the heating bill at home. Now Britain's richest woman - worth $US1 billion ($1.25 billion), according to Forbes magazine - her seven Potter books have sold more than 335 million copies worldwide.

Rowling said last month she believed she was unlikely to repeat the success of the series, but confirmed she planned to work on new books.

"I'll do exactly what I did with Harry - I'll write what I really want to write," she said.

The office of Rowling's literary agent Christopher Little was not available to comment.