Some of the modern mass-market books recommended upthread were written primarily for entertainment, which is fine as far as it goes, if it gets more people interested in finding out facts, but are regarded by Holocaust historians and educationalists as exploitative and inaccurate.
So I’d agree with some PPs who say move onto direct survivor testimony, and add to their list:
Night, Elie Wiesel
The Children's House of Belsen by Hetty Verolme
Dear God, have you ever gone hungry? Yosef Baʼu
I Was A Doctor in Auschwitz, Gisela Perl
This Way for the Gas, Ladies and Gentlemen, Tadeusz Borowski.
We Wept Without Tears: Testimonies of the Jewish Sonderkommando from Auschwitz, Gideon Greif.
For more fiction, The Shawl, by Cynthia Ozick. For young people, the Once series, by Morris Gleitzman, would be a better option than The Boy in the Striped Pyjamas. As would the Out of the Hitler Time series by Judith Carr, sometimes overlooked as children’s books. As well as telling a compelling story, they explore Carr’s own experience of the aftermath of the Holocaust and generational trauma in an accessible way.