Marcus appears to specialize in Ethiopian history (he has written biographies of Emperors Haille Selassie and Menelik, both available from Amazon.com). But before reading those you might want to read his History of Ethiopia.

Marcus traces the history of this fascinating but mysterious land back to earliest times, and takes it right up to the fall of Mengistu in 1991. Perhaps someday a revision should be done to add events that took place after that, but right now it is too soon; history is still being made in Ethiopia. Most of the book deals with the period after about 1850, but as I said, if you're looking for a good general history filled with facts and details, this is it.