Basically, I'd like to create a category permalink like example.com/books/adventure without having to make adventure a child-category because it will apply to other categories as well.
So the end result is:
The order is based on the two parent categories, with "items" going first, and "genre" going second.
And you could also search the categories separately to see all of the books, or all of the adventure items.
How can I do this?