During the writing of my first three novels, I have discovered the unbridled satisfaction of undertaking extensive, and in-depth research. I have been taken into areas which have shocked, broke my heart, incensed, reflected, celebrated, questioned..... The list is endless, and even for those not thinking of writing a book, I can only recommend finding a topic of interest to you, and diving as deep as you can.
Things I have discovered which have had a profound affect on me, I would commend you to research.
1. The tragic story of Oradour-Sur-Glane on June 19th 1944.
2. The Sandakan Death Marches, Burma, 1942 - 1945.
3. The involvement of IG Farben in the financing, and construction of Auschwitz-Birkenau.
4. The Canadian National Vimy Memorial, France.
There are many others, but I hope you discover your own.