From your description it sounds that in the referenced Books table your key column is Book ID and label is Book Name.
The app displays the user friendly label because for the app user, book name ( or label field ) is easier to understand than a typically cryptic Book ID ( key column). In the back end Google sheet the app stores the key column because it uniquely identifies the concerned record for the said book in the Books table.
If you wish to additionally capture the book name in the Books table in the Google sheet, you could create an additional [Books Name] column in the Sales table with a dereference expression in app formula such as [Ref Column Name in Your SalesTable That References Books Table].[Books Name Column Name in Books Table]
You may wish to keep this [Books Name] column in Sales table hidden because the app anyway displays the label name from the reference table.
Just in case you have not, you may wish to refer to following related articles