While The Actresses was based in the present day, her seven novels since have all been firmly rooted in the past. "I just love it," says Ewing, who regularly visits the British Library around the corner from her Bloomsbury home. "I realised how much you can learn from research. I locked myself away for months on end and learnt everything I could."
While numerous other authors have explored the Victorian era, Ewing has mostly concentrated on the preceding years. "I could have done a doctorate on it," she laughs. "I've made it my specialty to write about the late 18th century and the first half of the 19th century, which was a much freer time for women."
The Circus Of Ghosts follows on from Ewing's 2007 novel The Mesmerist, in which leading lady phreno-mesmerist Miss Cordelia Preston is embroiled in a sinister murder. Shifting from London in 1838 to New York in the late 1840s, she is reluctant to describe it as a sequel.
"It's a complete standalone book," she says. "I was lucky, having been told I had to write this book, I suddenly found out that anaesthetics came in a few years later and the medical mesmerists were finished."
A form of hypnotism involving animal magnetism, Ewing was initially dismissive of the much-derided practice before concluding that it boasted some noteworthy applications.
"When I wrote The Mesmerist, these two old actresses were going to be setting up a joke business but the more I studied it, the more I understood it wasn't a joke," says Ewing, who consulted Dr Bob Large from the Auckland Regional Pain Clinic on both books.
"I went to see him and he was very nice to me. He was a bit reserved at first until he realised I'd done all the research. Then he said he used it all the time in the cancer pain clinics. As soon as I met him, I realised I had to take it seriously. He got me on the whole trail of medical mesmerism and I'm interested in anaesthetics now."
Her first book located in the United States, Ewing ventured beyond her usual stomping grounds, spending several weeks searching out New York's few remaining historical remnants.
"It's not like London, where I just need to walk the streets," she says. "I actually wore out a pair of shoes trying to find the old bits of the Lower East Side and the docks."
However, she did make some intriguing discoveries. "They had running water, which is extraordinary," she says. "They certainly didn't have that in London, or the telegraph either. New York wasn't hampered by class and tradition, anyone could have a go. It was a bit like New Zealand. We would have understood that. It's the number eight wire thing in the same sort of weird way."
Like her past novels, The Circus Of Ghosts boasts some connections to Ewing's homeland. "I've mentioned New Zealand in every one of my novels because I like to think that we were there," says Ewing. "In my last novel, The Fraud, I've got Captain Cook going to New Zealand and Trespass is all about New Zealand." In the new book, Cordelia and Gwenlliam cross paths with trailblazing Kiwi actress Colleen Ray, whose colourful exploits Ewing stumbled upon in a newspaper archive.
"The book takes in the goldrush and travels across America. While I was reading up about that, to my delight I found a review of The Bandit Queen, starring Miss Colleen Ray from the Royal Theatre in New Zealand. She has a few adventures in this but she was a real person, as was Gallus Mag, one of the gang leaders who used to bite off people's ears and keep them in alcohol. When I come across people like that, I just feel so lucky."
The Circus Of Ghosts (Hodder & Stoughton $39.99) is out now.
