Handbooks and guides typically offer the reader an opportunity to survey and learn about a broad field of scholarship in a comprehensive way. They give a snap shop of the discipline covered at the time the book is published. Unlike a retrospective collection of articles, these guides are written or edited as a planned and intentional effort, with chapters and sections interrelating and complimenting one another and thereby tend to be more readable and integrated research works.
Language in Africa by 
The Languages of Africa by 
Practical Phonetics for Students of African Languages by The following databases are good places to go to start researching your topic. Each database has a slightly different focus, so you might need to try out a couple of databases before finding one that returns the sort of results you are looking for.
The following are links to specific academic journals which are published and peer-reviewed by experts in the field. If you want to narrow your search to a specific topic, country, or issue, finding a journal that publishes in that area can help.