I find this question lands differently after reading some incredibly detailed diaries and testimonies that aren't fiction. Sure, 'Night' by Elie Wiesel is the canonical first-person account, and for good reason. Its sparse, haunting prose is almost unbearable. But one that shook me in a different way was 'The Diary of Rywka Lipszyc'. It was found in the ashes of Auschwitz and published much later. It’s a teenage girl’s writing from the Łódź Ghetto, filled with the daily grind of hunger and fear, but also her profound religious faith and literary ambitions. That juxtaposition—her intellectual life persisting in that hell—is a unique lens.
Another angle is from the perpetrators, which is a difficult but necessary perspective. Jonathan Littell's 'The Kindly Ones' is a fictional memoir of an SS officer, a monstrous, exhaustive dive into the bureaucratic mentality of genocide. It's not for the faint of heart, and it’s controversial for its graphic content and interiority, but it forces you to stare into that particular abyss. For a more focused, real-world bureaucratic view, Hannah Arendt's 'Eichmann in Jerusalem' remains essential, though it's reportage, not a novel. It grapples with the ‘banality of evil’ concept through direct trial observation, which is its own form of eyewitnessing.
Then there are the accounts from those on the periphery, which can be uniquely illuminating. 'The Book of Aron' by Jim Shepard comes to mind. It's a novel based on the real-life pediatrician Janusz Korczak, but told through the eyes of a young, cynical, and morally compromised boy in the Warsaw Ghetto. That choice of narrator—a kid who isn't noble, who steals and lies to survive—feels brutally honest. It strips away any sentimental veneer and shows how the ghetto corroded ordinary morality. That’s a perspective you don't get from every memoir.