If you are going to be the main poster initially and for the foreseeable future, probably a genre you are interested in.

I’d also suggest making the post in the new/niche community and then cross post it into a more general/main/popular community (ex. Books).

Good luck on your new community :)

I don’t see a need to “push” a genre, if this is a general book discussing community.

Go with “whatever you enjoy reading about” and that should do it.

I personally enjoy science fiction and fantasy fiction. Currently reading forgotten realms fantasy.

Has to be something you care about. Starting a community is a lot of effort so you’ll need a definite interest in the subject.