Book on the psychology of killing in warfare, and the impact of having killed on the minds of the soldiers, especially focusing on the data about how few soldiers, historically, actually participated in killing, and why in modern conflicts, the %age of soldiers who fire has gone from 20% to 90% (Skinnerian conditioning). Grossman alleges that only 2% of people can kill, even with the justifications of a combat situation, without severe psychological stress. The trauma of combat and killing then leads to PSTD, especially without social support and ways of reintegrating the veteran into society.
It is true that writers often fail to adequately portray the impact of killing, especially since they are unaware of the significance of posturing, and often gloss over how psychologically traumatic it is to kill at close quarters. Warriors and such in books/movies then, often seem to be in that one in fifty, without acknowledgement of how unusual they are.