If your item (taxonomy or CPT) can stand alone by itself as a page of detail, it should be a CPT. If it can only exist as a group of things, it’s a Custom Taxonomy.
For instance, I don’t feel Authors is a Custom Taxonomy of Books. They’re both Custom Post Types and related to each other because they stand alone as unique items.
However, Genre is a Custom Taxonomy, because while you can define it, give it a featured image, a color on your website, a nice description and an icon, it doesn’t exist on it’s own without a list of books or authors in that Genre.
