Africans traditionally are said to have no sacred writings. Where can one find important information regarding these religions?

Traditional religious practices in Africa are by no means lost to the world, in spite of there not having been any written sacred texts.  There is oral tradition and also, African religions have been studied by scholars for a very long time, for example, anthropologists, theologians, and linguists. 

People practiced their religions and handed them down to succeeding generations well before there was writing. Even once man began to write, few people were literate, so...

Traditional religious practices in Africa are by no means lost to the world, in spite of there not having been any written sacred texts.  There is oral tradition and also, African religions have been studied by scholars for a very long time, for example, anthropologists, theologians, and linguists. 

People practiced their religions and handed them down to succeeding generations well before there was writing. Even once man began to write, few people were literate, so most religions were passed on this way for a very long time after writing was invented.  Thus sacred texts, for example, creation myths and rituals, became part of an oral tradition, just as they have been handed down by Native Americans.

Since Africa has been colonized, there have been people interested in studying religion in Africa, and in their studies, they have recorded what they have learned. Anthropologists have done many studies, and I have provided a link to an overview of one, as well as a few other websites that are reputable, for more information and suggested readings.  Missionaries, too, recorded some of what they learned about African religions, and this is another source of knowledge. Linguists study language all over the world, and in their studies of African languages, they also have gleaned knowledge of African religious practices. 

It is remarkable how successful oral tradition is at carrying on a religion. And fortunately, we have had enough people interested enough in pursuing knowledge of African religions to have a reasonable body of work on this subject.