There's a good book called 'The Scramble for Africa' by (Thomas?) Pakenham - it looks at the European Empires' takeover, but has a lot on what was there before, and on those areas and countries (Like Ethiopia) which defended themselves. I never realised that the Ethipian Army inflicted the greatest defeat on any European Army by and African one when they told the Italians where to get off - that's why Mussolini was so keen to smack it back to them in the '30s (mustard gas and all).
Any other book tips would be appreciated - I find it fascinating to read about these civilisations which are so hidden in our school history. It helps counter the 'they all lived in mud huts 'til we arrived' school of thought.