Hi Toomas, sorry for the confusion.
Where do I need to put this code exactly?
Please put them directly below your code to register meta box. Or in the theme’s
What “location” in your code means? “Taxonomy_2” is the name of a custom taxonomy field?
I should have mentioned you need to change the “location” to the name of the custom post type created by CPT-Onomies plugin (in your case, it’s “books”) and “taxonomy_2” – the ID of the taxonomy field (in your case, it’s still “taxonomy_2”).
Please try again and see if it works.