How to competently build the structure of the subscriptions table?

Question

Web development

Good morning!
I have such a problem: It is necessary to implement a system of subscriptions to categories and books.
After registering on the site, the user is greeted by a page in which he can subscribe to categories, well, or to books that interest him.
The table structure is as follows at the moment:

1. id
   
2. user_id
   
3. category_id
   
4. book_id
   
5. created_at
   

Question: In what form to write down the data in the table?
How would you make this entire system yourself?
I really want to do it right!
Thanks and have a nice day!