The book_id field is going to used as the primary key for the books table.
- Click on the book_id field in the books table to select it.
- Look at the inspector, at the bottom is a tabbed panel. Make sure the tab Options is visible. Click the tab if it isn't visible
- Look for the checkbox Primary Key and check it
In the main window a P icon will appear on the field. This indicates that it is used as a primary key.