It’s a philosophical book about forgiveness
The premise is that it’s a personal account from the author, who was a Jew living in concentration camps in WW2. A nazi soldier, on his deathbed, calls for him to apologise for his atrocities, the families he tore apart and the murders he committed, including tying Jewish protestors inside a building, lighting it on fire and shooting any escapees.
Then, the soldier pleas for forgiveness from Simon, and says that he can’t rest peacefully if he isn’t forgiven.
Simon leaves the room without a word, and the book is a call to all other philosophers and ethical thinkers about if what he did was right. Most agree, and say that it isn’t his right to apologise for other victims.