If you are showing author data in the parent form, then there should be a
sub-form for books. The data source for the subform should be an inner join
of the book table with the book-author junction table on book ID. The linking
field from parent to child must be author ID.

Conversely, if the parent contains book data, its subform shows authors. The
data source for the subform is an inner join of the author table with the
book-author junction on author ID. The linking field from parent to child
must be book ID.

Note that entries into the subforms must allow inserting links to existing
child records as well as adding new ones, to allow for the case of adding a
new book by an existing author, or adding an author to an existing book.

Hope this helps.