Conversation so far:
what kind of genres are there for books
Books are either fiction or non-fiction. Non-fiction books contain factual information. Fiction books contain a story made up by the author, such as romance or children's books.
what is the most popular genre for books
According to Statistica, the most popular genre in the U.S. is Mystery/Thriller/Crime books. Nearly half of their respondents said they had read books in this genre in 2015.

Question: what is the best-selling in mystery
Self-contained rewrite: what is the best-selling book in the mystery genre