See direct (instead of indirect) info about rented book in my "actors_directors" talbe dropdown list - Access365
You can make foreignKeyBook field as a compobox with coulumn count = 2,  column widths =0"1", and row source is a query of books.index and books.title in that order

Or you could easily make this in the beginning while designing the checkout table by setting the data type of field ForeignKeyBook to lookup withered.. and following the lookup wizard window