1 - Essays and speeches on the eve of the civil war - Pro - R. Raphall , R. Illowy, Anti - M. Heilprin, Dr. Einhorn.
2 - Einhorn's article is particularly well argued. Regarding this point:
Only on one point has Dr. Raphall shown a friendly disposition towards the negro; at the expense of his holiest duty, he has failed to call to the attention of the Jewish slave-holders that they must have their slaves circumcised.
I have heard of (but not seen - nor can I find) a Teshuva to the Noda B' Yehuda [?] sent by a Jewish slave-owner in the south. I believe the NY paskened like Dr. Einhorn that circumcision is required. The end of the story is that the slaves heard of this plan and revolted. See also J. Schorsch "Jews and Blacks in the early modern world".
3 - R' Yaakov Kamenetzky said (in a lecture) that the בני כנען also have a צלם אלהים and it is therefore absolutely forbidden to embarass or discriminate against them in any way.