Frederick Douglass, "Letter to His Old Master" (1848) from Frederick Douglass: Autobiographies:
In grad school, I took a class in the History Department called "Important Primary Sources in American History"--or something to that effect. So in one week, I devoured something like 1000 pages of slave narratives (the Library of America collection, actually).
But it never gets any easier. This short, humorous, cutting letter from Douglass to his former owner doesn't dwell on the horrors of slavery at any length. It mentions them in general (being robbed of education, etc.) or names particular events (being dragged to market, etc.) only sparingly; most of this letter is taken up by the joys of freedom.
And yet, those joys are so obviously contrasted to the crushing pains of slavery that this letter can get me teary eyed. Douglass's great skill here might be in how quickly he can veer from cutting and acid humor to straightforward pathos.
Edit: If you read this letter, make sure to read the LoA headnote, which tells the story of some strange meetings between Douglass and the family that formerly owned him. It's a great reminder of how strange and intimate the institution of slavery really was.