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.
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.