Currently, list filtering on Hardcover allows users to show books that exist within a specific list (e.g., filtering a prompt or search by books on the "Owned" list). However, there is no way to filter by exclusion—showing books that are not present on a specified list.

Use Case:
When browsing large lists (like "To Read," curated award lists, or recommendation prompts), users often want to find books they haven’t acquired or read yet. For example, filtering a "Want to Read" list to exclude books on the "Owned" list instantly reveals which books need to be bought or checked out from the library.

Proposed Solution:

Toggle / Operator Support: Add an exclusion toggle (or a "NOT" / "Is Not On" filter option) alongside existing list filters.

UI Example: In the list filter dropdown, allow clicking a filter once for In [List Name] (inclusion) and clicking again (or selecting a "Exclude" icon/option) for Not in [List Name] (exclusion).