More general background reading:
The classic is Basil Davidson’s “The Story of Africa” (1984), describing African history from the early years through the development of individual kingdoms and the advent of colonial expansion, to the increasing assertiveness of African people and independence.
Less relevant for the Horn, at least Ethiopia, is “The Scramble for Africa: White Man’s Conquest of the Dark Continent”, written by Thomas Pakenham in 1991. Yet, a thrilling account of Africa’s colonial history it is.
The travel guides: