Filters to display books including genres and excluding genres

Scenario:

  • A website contains a list of about 10,000 books.
  • A user would like to discover a book he could enjoy by genre (there are 15 of them)
  • He ticks a box to show only books that include the genre "thriller"
  • Too many books appear and also contain the genre "horror" (the user does not like the horror)
  • The user would like to be able to exclude "horror" from the thrillers list returned.

I am looking for a non-awkward way to allow the user to exclude genres that, preferably, do not clutter and interfere with the "include" list (it's not a good idea). is a simple and solid UX that should be considered as a main filtering method). I would also like to avoid having to list the 15 genres again in another checklist. There are other types of common filters that are part of the same user interface (length, sort, search by keyword), but I only need help for the above.

My only thought was to maybe keep the initially masked exclusives unless they were activated.

The UI area is a sidebar with a maximum height of about 250 pixels wide and can scroll independently.