New Zealand author Heather Morris' 2018 debut novel, The Tattooist Of Auschwitz, based on the real-life story of a Holocaust survivor, has sold more than three million copies worldwide.
It has also been criticised by the Auschwitz Memorial Research Centre as containing "numerous errors and information inconsistent with the facts, as well as exaggerations, misinterpretations and understatements".
Now Morris, 66, has taken one of the most challenged parts of her book - the story of Cecilia "Cilka" Klein, a 16-year-old Slovakian Jew who becomes the sex slave of the concentration camp commandant - and made it into a sequel, Cilka's Journey, that has already sparked controversy and even condemnation.
Cilka appears in The Tattooist Of Auschwitz, which is based on the story of Lale Sokolov, a Jewish prisoner of Auschwitz who served as the camp's tatowierer (tattooist). He fell in love with his future wife, Gisela "Gita" Furman, when he tattooed her prisoner identification number. After the war, they found each other again, married and moved to Melbourne.
Before Mr Sokolov died in 2006, he told his story to Morris, who also lives in Melbourne and is married with three children.
Morris, then a hospital worker and aspiring screenwriter who had never written a book, was introduced to the 87-year-old by a friend of his son. He was then mourning the death of Ms Furman and wanted to tell their story to somebody - preferably not Jewish, as most Jews would have their own connection to the Holocaust - before he too died.
Morris recalls Mr Sokolov mentioning Cilka, a friend of his wife's in Auschwitz-Birkenau camp, as "the bravest person he ever met - not the bravest girl, the bravest person" for having endured what she did to survive.
She had played a part, he said, in saving his life when he ran afoul of the camp authorities, by using her position to intercede with the commandant.
"I knew I had to find out more and tell her story, and I am very honoured to have written it," she says over the telephone from Britain, where she is on a book tour.
In the sequel, Cilka is freed in 1945 by the Soviets, only to be sentenced to 10 years in Vorkuta, a brutal Siberian gulag north of the Arctic Circle, on the charge of collaborating with the Nazis.
"I found myself getting so angry and impotent because there's nothing that my writing her story can do to take away the horror that was inflicted on her," says Morris.
"I don't think it is exploitative to call it out for what it is, which was rape, pure and simple.
"To suggest this 16-year-old was in some way a seductress who prostituted herself to the commandant in Birkenau just makes me really angry. She did what she had to do to survive. It does not make her a prostitute, it makes her a rape victim."
The character of Cilka is based on Cecilia Klein, who died in 2004. Morris says she interviewed other Holocaust survivors who knew Ms Klein, engaged professional researchers in Moscow to find out about the details of life in Vorkuta and travelled to Slovakia twice.
Morris notes in the book's introduction that "it is a novel and does not represent the entire facts of Cilka's life". Ms Klein's husband, Mr Ivan Kovach, whom she met in the gulag, does not appear in the novel. Cilka falls in love instead with a prisoner called Alexandr.
The Guardian reported earlier this month that Ms Klein's stepson, Mr George Kovach, had denounced Morris' novel as "lurid and titillating".
He took issue with Cilka being presented as "the mistress of not one but two high-ranking SS camp commanders" and as having allegedly stolen drugs from the Vorkuta camp hospital.
Mr Kovach said Morris had asked him to contribute pictures and an afterword to Cilka's Journey, but that he withdrew from the project and later told her publishers the novel might infringe on his rights.
In a report last year, Auschwitz Memorial researcher Wanda Witek-Malicka criticised numerous parts of The Tattooist Of Auschwitz, arguing that Cilka's relationship with the camp commandant could not have been maintained over such a long period and would have resulted in an "accusation of race dishonour".
Morris maintains she heard or read testimonies from people who said Cilka had been made leader of Block 25, where female prisoners spent their final hours before the gas chambers, and that this was how the camp commandant had kept up their secret relationship with few witnesses.
She says: "Lale and every other survivor I spoke to said no two people would have gone through Auschwitz-Birkenau or any camp the same way. There are academics and historians who tell the story of the whole Holocaust - I just tell individual stories. As to inaccuracies, it comes down to memory and history and where those two dance and step.
"I honour (the Auschwitz Memorial) and I respect them for what they do. I think there is room for both of us and what we are doing in keeping the story and narrative of the Holocaust alive."
• The Tattooist Of Auschwitz ($17.95) and Cilka's Journey ($29.95) are available at major bookstores.