David Gillham vividly remembers the first time he read Anne Frank’s The Diary of a Young Girl : “I just burst into tears in the midst of reading it. ” He was then a 25-year-old creative writing student, now nearly four decades later he is a best-selling author who has just published a novel, Annelies, which imagines Anne surviving the Holocaust.

“A lot of people look at Anne Frank’s diary and think: a talented teenager who had such potential and it was all lost ,” Gillham explains, from his writing studio in Western Massachusetts, where a portrait of Anne Frank hangs on the wall behind him. “I was thunderstruck by her ability when I first read her diary, decades ago, and as a writer of fiction, I always knew that that I must one day write about her.”

The success of his first novel, City of Women in 2012 gave him the chance to embark on his Anne Frank book. He spent six years researching the novel, visiting Amsterdam often to familiarise himself with the Frank family’s home in the Rivierenbuurt district before they went into hiding, as well as the old Jewish quarter. At what is now the Anne Frank House on the Prisengracht, where the family hid, he was given access to areas of the house that tourists do not visit. He followed Anne’s path after she and the others in the house were betrayed, to the Gestapo building in the south of Amsterdam, to the remains of the transit camp at Westerbork, to Auschwitz-Birkenau where her mother died, and to Bergen Belsen, where Anne and her sister Margot succumbed to typhus.

“I was trying to craft a novel with an authentic tone that not only told a story of what might have been had she survived, but to do so while treating her legacy with the dignity, integrity and respect it demands. In a way, Anne Frank became my muse during the creation of this book. I followed her spirit as best I could into the very darkest period of history, when the world descended into madness.”

Annelies alters some facts to suit its own counterfactual historical narrative.The novel begins in April 1945, imagining Anne emerging from a Nazi death camp in Germany alive but traumatised; returning to a liberated Amsterdam to live with her father, Otto, before eventually emigrating to New York, following the publication of her best-selling Holocaust survivor memoir. Her father remarries —not to his actual second wife, but one imagined by Gillham. The book ends in 1961 with Anne determined on the path of tikkun olam — repairing the world through her writing.

The narrative skips back and forth between the diary years in Amsterdam, before the Franks were deported. Where possible, Gillham remains loyal to historical accuracy, using actual incidents from the diary to recreate dramatic moments in the fictionalised tale.

Otto’s relationship with Anne is the central focus of Gillham’s counterfactual historical tale.

“There is no doubt that when he came back from the camps Otto was a devastated and broken man. As a father, Otto was very close to Anne, and yet there was so much about Anne that he did not know until he read the diary.”

Gillham believes it was the poignant and hopeful message that his daughter’s words provided that helped Otto Frank pursue a life where meaning could be found after the trauma of losing his entire family.

“Anne was used as an inspiration and a beacon of hope internationally.Such a positive narrative really fitted in with the kind of man that Otto was. He didn’t want to dwell on the horror of the past, but to move forward instead.”

Anne Frank’s diary may never have been discovered if it wasn’t for Miep Gies, an employee who worked for Otto Frank’s pectin and spice company. Gies also helped to hide the Frank family.

“After the Gestapo came and arrested the Frank family the diary was just left on the floor, and Gies picked it up, didn’t read a word, and left it in her desk drawer. But when it turned out that Anne wasn’t going to return, Gies took the diary from her desk, put them in front of Otto, and said: here is the legacy of your daughter,Anne. And the diary saved Otto’s life.”

When he originally published the diary in 1947, Otto Frank was extremely careful and cautious in his editorial process: omitting a number of unflattering passages Anne wrote about her own mother,Edith.

Only in 1995 was the uncut original diary published. It revealed a more candid Anne Frank who confesses in one diary entry that she “cannot stand Mother [and] it’s obvious that I’m a stranger to her.”

“Daddy usually comes to my defense,” she writes elsewhere.

“Guilt is something I really focused on exploring in this novel,” says Gillham, “particularly the idea of survivors guilt.”

He stresses that the relationship eventually mellowed when the Franks were deported from Amsterdam, something that people who met the Franks in the camps attested. “All of that criticism,bitterness, and mother-daughter problems that had existed beforehand just dissolved into nothing. Anne’s mother really came into her own when she was with Anne and Margot at Auschwitz -Birkenau; trying to protect her girls as best as she could in this terrible environment.”

No one knows for certain who betrayed the Frank family. In Gilham’s novel there’s a scene set at the top of the Empire State Building - where Elizabeth “Bep” Voskuijl — another employee of Otto Frank —says that it was her sister Nelli who betrayed the Franks to the Gestapo. The idea came from a 2015 book written by Bep’s son.

What does Gillham hope to achieve with his book? “I wanted to underscore what Annelies Marie Frank and her work have come to represent to me and to millions of others — hope.

“In what is probably her most famous diary entry, Anne Frank declares that, even in the midst of the world’s brutality, she still believed that people were good at heart. This was entry made while she and her family were still in hiding, before their betrayal and arrest.

“The question I’ve heard asked is, “Would she have still felt the same after Auschwitz?” I thought it was an important question. A question that needed to be answered with same hope for the future that themes her diary. This is what I have attempted to do with Annelies.”

Annelies is published by Fig Tree (£14.99)