You would have to redesign the table the is in the subform so that you would enter a the borrower's name using a drop down. You would have to create a table of borrower's, and then create a look up to the borrower's table from the junction table in your subform.
You would thus have:
(other book info...)
(other borrower info...)
tblLoan - In your subform
BookID - look up to tblBook
BorrowerID - look up to tblBorrower
(other loan info...)
This would allow you to look up borrowers in the book table form, and to look up books in the Borrower table form as well.