For me, it was "Vimy" by Pierre Berton, "Sounder" by William H. Armstrong, and "A Whale For The Killing" by Farley Mowat.
Each of these books left scars on my psyche that while healed, have never been forgotten. I read these when I was very young, and have never forgotten the impact that each one left on my impressionable mind.
I suppose the common theme through each of these is just how ugly humanity can be, towards himself and the world he lives in. I always hoped that I wouldn't contribute to the ugliness that I had experienced through these books.