Tannenbaum's essay is classic in the field of comparative slavery. Personally, I enjoyed Tannenbaum's essay, even though some of Tannenbaum's thinking is archaic there are subjects he touches where he anticipates historical trends. Some of the historical trends he anticipates are slave raids and the study of the middle passage, illegal slave trade, and acquiring whiteness. Slave and Citizen is a great jumping-off point if you want to situate yourself in the historiography of combative slavery.