The Black Panther comic has its own unique fictional universe and narrative that is not related to the Black Panther Group. The comic's creation was based on comic book concepts and storytelling rather than any direct connection to the group.
Cultural exploration is another theme. Since Wakanda has such a rich and unique culture, fanfictions will often incorporate elements of it. This can range from the traditional ceremonies to the advanced technology. In the context of a slash relationship, it might show how the two characters interact with their cultural heritage while being in a relationship that may not be considered traditional within Wakanda.
One great 'Black Panther' fanfiction is 'The New Wakanda' which explores a Wakanda that has to face a new, unseen threat. It has great character development for T'Challa and his sister Shuri.
The Legend of Black Panther is an audio novel that can be listened to for free. However, the search results did not provide clear information on which platform could be listened to for free. Therefore, I don't know the specific platform for free listening to books.