Hi,
Thanks for the details.
I suppose you only want to return the taxonomy terms from the book related post type. Since you added details, I have corrected the configuration:
– The post tags and categories search was enabled, I turned off that here: https://i.imgur.com/oZloclC.png
– I turned off every front-end filter, since it was not in use at all: https://i.imgur.com/8aYzwfL.png
Now it should only return the tags from the two taxonomies.