Filters to show books including genres & excluding genres

Scenario:

• A website has a list of ~10,000 books
• A user would like to discover a book they might enjoy via genre (there are 15 to choose from)
• He ticks a checkbox to show only books that include the genre "thriller"
• Too many books appear that also contain the genre "horror" (the user doesn't like horror)
• The user would like a way to exclude "horror" from the returned list of thrillers.

I'm looking for a non-clunky way of allowing the user to exclude genres that preferably doesn't clutter or interfere with the "include" list (it is a solid, simple UX that is to be considered primary method of filtering). I would also like to avoid having to list all 15 genres again in another checklist. There are other type of common filters as part of the same UI (length, sort by, keyword search) however I only need help with the above.

My only thought it to maybe keep the excludes initially hidden unless activated somehow.

The area in the UI is a full-height sidebar ~250px wide and is independently scrollable.