Thinking specifically of mercenary involvement in places like Rhodesia and the DRC, but anywhere will do
There is a chapter in Odd Arne Westad’s “The Global Cold War: Third World Interventions and the Makings of Our Times” that I believe deals with South Africa and Rhodesia. Might be a useful starting place.
With regards to the DRC I assume the mention of mercenaries means you’re most interested in the Congo Crisis? You might try Lise Namikas’s "Battleground Africa: Cold War in the Congo, 1960–1965."