Early on in ''Islam's Black Slaves,'' his history of slavery in the Muslim world, Ronald Segal cites some estimates. One scholar puts the rough total at 11.5 million slaves during more than a dozen centuries, and another at 14 million. We will never know the precise number, of course, but it is striking that these two figures neatly bracket many scholars' estimates for the much-better-documented Atlantic slave trade. So why in the West today do we generally pay so little attention to Islamic slavery? One reason, suggests Segal, a South African-born editor and the author of ''The Black Diaspora,'' is that in the Muslim world slavery never became the publicly fought moral and political issue that it did in the United States and Europe.