Grass was a young boy when World War II started (he was 11 years old when the war started). He is (if you don't know) German. So, he was old enough to have some idea what was going on, but too young to have any degree of control over his life. He was just another ordinary German civilian caught up in what became one of the greatest tragedies in world history.
This book, his first novel, written in the late 1950s, is obviously a reaction to his life during the war. It is not directly autobiographical because it is told using metaphors and many things happen which (in all likelihood) never happened to young Mr. Grass. However, the metaphors "work" in a way that perhaps gets at the emotional truth of what happened.
Frankly, this book didn't quite succeed for me. I do not rate it as one of the great novels of the 20th century. As an historian I know a great deal about the facts of the war. What was it like to be a young boy in Nazi Germany, seeing your world literally destroyed before your eyes? Seeing your older cousins going off to war and never coming back? And then, learning after the war, about the monstrous crimes committed by "ordinary Germans"? I'm not at all convinced that this book really gives you a good sense of that. For me, at least, it's just too absurd.
Call me a foolish realist. Tell me you can only capture the horror and chaos of seeing your city burned to the ground, your whole life torn apart through forces beyond your control, through absurdity and metaphor. I'd rather Grass told the truth, or told a truthful story. To my mind, "The Tin Drum" is not a truthful story.
Still, I can see why some people like it. It has quality both in writing and in the senses it brings to life through the pages of the story. Certainly it has left an impression on me even after many years. One thing is certain, I have no desire to read this book again.