Night By Elie Wiesel
In terms of impact per word, this thin volume is by far the hands-down winner of everything I have read. If it's possible for words to overpower guns and knifes, this thin volume may be the closest thing. It's only about 100 pages, and you can finish it in less than a day, but the effects will be with you forever.
The amazing part about this book is the author's ability to put you in his shoes, his mind, while the worst of humanity unfolds, and how that breaks everyone down to the product of the worst of humanity. I especially appreciate his frankness when he penned the following line
"If only I could get rid of this dead weight ... Immediately I felt ashamed of myself, ashamed forever."
The dead weight is his father.
This line is the result of the camps impaling his soul, twist the knife, then pull it out, and let the contents pour out without filter or censor. It gives us a glimpse of what we will turn into when we are no longer human.