This is one of the best reference works for African history, and it is free. You can download the entire series (thousands of pages) through this link. It covers ancient history through the end of the twentieth century.
When you are using this work to get leads for future reading, you will have to compare the citations in the chapter your are reading with the bibliography at the end of the volume. The full citations are only available in the bibliography.
This bibliography is a work in progress, so it doesn't have as many entries as some of the other resources. However, it is very high quality, so it is worth checking to see if you can find stuff related to your topic.