I'm sure other posters with more Africa-specific knowledge can point you in the direction of reading around general Soviet policy in Africa, but regarding specifically Soviet intelligence's engagement there, I'd strongly recommend The World Was Going Our Way: The KGB and the Battle for the the Third World by Christopher Andrew and Vasili Mitrokhin. Andrew is a world-class intelligence historian, and Mitrokhin is a former KGB archivist and defector. The book examines Soviet policy and intelligence operations around the world, including a couple of chapters looking at Africa specifically. There's some interesting detail in there around the KGB's relationships with the African National Congress and South African Communist Party, and their support for uMkhonto we Sizwe's armed campaign.