Although '' Once '' is a work of fiction , Gleitzman was inspired by the story of Janusz Korczak , the events of World War II , and Hitler 's attempt to exterminate the Jewish population of Europe . As research for the novel , Gleitzman read several books about and by young people in the Holocaust , including '' The King of Children '' by Betty Jean Lifton ( a biography of Janusz Korczak ) , '' Salvaged Pages : Young Writers ' Diaries of The Holocaust '' , edited by Alexander Zapruder , '' The Hidden Children '' by Jane Marks , '' Words to Outlive Us : Eyewitness Accounts From the Warsaw Ghetto '' , edited by Michał Grynberg , '' Witness : Voices From The Holocaust '' , edited by Joshua M. Greene and Shiva Kumar , '' A Childhood '' by Jona Oberski , '' Maus '' by Art Spiegelman , '' The Diary of a Young Girl '' by Anne Frank , '' Born Guilty '' by Peter Sichrovsky , '' The Hidden Children '' by Howard Greenfeld , '' Children of the Ghetto '' by Sheva Glas-Wiener , '' Konin : A Quest '' by Theo Richmond , '' The Boys '' by Martin Gilbert , '' Flares of Memory : Stories of Childhood During The Holocaust '' , edited by Anita Brostoff with Sheila Chamovitz , '' Yiddishland '' by Gerard Silvain and Henri Minczeles , '' Children With a Star '' by Debórah Dwork , and '' Ghetto Diary '' by Janusz Korczak
exterminate the Jewish population
The Holocaust
Once (Morris Gleitzman novel)