Well, I would hope that it's fairly self-evident that the way, e.g., Robert E. Howard wrote about and portrayed Black people in his fiction (in the Conan stories and elsewhere) was pretty terrible, and unfortunately you don't need to dig all that deep to see it. Which is not to say you shouldn't read or enjoy Conan - Saunders certainly did both! - only that it's a notable flaw, and one he shared with a lot of his contemporaries and followers, including folks in the fantasy game hobby.
As far as where to start for alternative views, I began my own research for this with the Wikipedia article on Abyssinia and followed several rabbit holes from there, including some dives into the specific history and folklore of Ethiopia and Eritrea. But even more relevant is probably the work of the Sword and Soul writers, which I would take as a pretty clear picture from POC of how they'd like to see themselves depicted in adventure fiction. Imaro itself is a great read; if you want a sampler of what the field looks like more recently, there's an anthology called Griots (which I confess I haven't read yet, but comes highly recommended) that also got a follow-up volume a few years later, where you can find the work of the major voices in the genre. There's a lot out there, and it's very cool!