There is the scifi stackexchange (https://scifi.stackexchange.com/ https://scifi.stackexchange.com/). Otherwise, subreddits for the particular genre/author/book

There is the scifi stackexchange (https://scifi.stackexchange.com/ https://scifi.stackexchange.com/). Otherwise, subreddits for the particular genre/author/book exist for fairly popular books nowadays.