I would never take seriously an examination of the Civil War that used the term "n----r" except in historical contexts. "Jap" and "Nip" and so forth, similarly, have no place in a serious discussion of the issues. His choice of diction shows he is nothing more than a hardened old bigot, still living in the 1940's.
On that we can agree.
If you seek the "TRUTH", then your sense of right and wrong are informed by the thoughts and actions of the people who were there, at that specific historical moment.
If "racial slurs" keep you from considering accounts and documents out of moral outrage, you ARE the sort of person who could claim that Truman was wrong to use the bombs, and you would be wrong.
PS, never EVER read Mark Twain, it would offend your superior morality.