Books are shelved in general subject categories using the Library of Congress classification system. You may want to supplement your use of the catalog book search with browsing shelf areas for your topic.
Books on African history are shelved on the library's upper level from DT 1 through DT 3415.
For Specific Areas:
Uganda - DT 433.2 - DT 433.29
Rwanda - DT 450 - DT 450.49
Nigeria - DT 515 - DT 515.9
Zaire - DT 641 - DT 665
Image from: David Rumsey Map Collection