Yes, you guessed right. In tbl_Distributions, book_id and bookstore_id are foreign keys for book_id in tbl_Books, respectively for bookstore_id in tbl_Bookstores.

Please kindly answer a rather theoretical question:

in what normal forms (..., 3NF, BCNF, 4NF, 5NF, 6NF, etc.) (why, and how to maximally normalize) the following example of a simple database formed by solving the many-to-many relationship between the tables tbl_Books and tbl_ Bookstores:

The [out_of_stock] field is of logical type (Yes/No).

The tbl_Books table stores book titles.

tbl_Bookstores (bookstore_id, bookstore_name)
tbl_Books (book_id, book_name, out_of_stock, sale_price)

tbl_Distributions (book_id, bookstore_id, total_cost)

In my opinion, the database is at least in normal form 3, in which case most documentations stop normalizing it, because for normal purposes it works as is. However, I would like us to do a deeper analysis on this simple example, because the mathematical theories involved in the study of the next normal forms, BCNF, NF4, NF5, etc. that the average user like me has access to are a bit too abstract even for a math teacher.
Thank you very much again!