You can always create your own custom database tables and access them with or without the WordPress wrapper.
However in this case, it sounds like you want Custom Post Types
I think you may have a bit of a misunderstanding as to what a “post” or “custom post type” is in WordPress. Though 100% honestly, “Custom Post Type” really is a misnomer, and a better name would be “Custom Data Type”.
You can code in your own Custom Post Types or use the widely accepted and used plugins CPT UI and Advanced Custom Fields
You can add custom taxonomies (which is just the word we use for “Categories” when referring to Custom Post Types) as well as add custom fields which will allow you to add meta information to each one.
Essentially, either with the plugins or manually, you’ll create a custom post type called “Book” and register taxonomies/terms and or use custom fields to add the relationships and entities to each one.
This is very easily doable in WordPress – let me know if you need me to expand on anything further!