“All quiet on Western front”, as well as the follow up novels “Three Comrades” and “Arc de Triumph” by Erich Maria Remarque.

I’ve read it when I was in my late teens / early twenties and it sort of changed my outlook on things. My then very naive and straightforward view of the world where right and wrong are obvious was shattered. It’s kind of made me sad and that sadness has remained with me ever since.

Since that time it’s been my answer to a question about a book that has had the most impact on me.