my $author = My::DB::Authors->retrieve($author_id);

# method add_to_books is created for you by has_many
$author->add_to_books({
                       col1 => value1,
                       col2 => value2,
                       ...
});  # author_id will be filled in automatically

my $books = $author->books;
while (my $book = $books->next) {
  ...
}