A few weeks ago I stumbled on a book titled
Oral Literature in Africa by Ruth Finnegan, which was originally published in 1970, and later revised. It's considered a classic in the field, and has since been made available as a free e-book
here. I've been able to commit some time to it, and can vouch for the chapter on poetry (as well as the rest) being well worth checking out.
I thought I'd share as I figured there'd be at least a few people interested in taking a look. And if you're aware of any other titles on the subject (ideally pre-modern), I'd be interested in hearing about them. I'm mainly wanting to get at the literature itself.