The Chris Peers book CENTRAL AFRICA: Tribal and Colonial Armies in the Congo, Gabon, Rwanda, Burundi, Northern Rhodesia and Nyasaland, 1800 to 1900 is in print and available on Amazon for ~$33 US.
COLONIAL ARMIES IN AFRICA 1850-1918: Organisation, Warfare, Dress and Weapons (Armies of the Nineteenth Centu) by Peter Abbott is available on Amazon at ~$125 US. It appears to be out of print.
It's a shame, the Foundry books are nice.