Insert value to database in CodeIgniter; I have array <input> tag

Question

How can I insert a value to database having an array values in my <input> tag. I'm making online library system; this part is I'm adding a book to database sometimes a book have more than 1 author.

Now I use implode to insert but the 2 authors are sharing in 1 one column. What I want they have to be in different column when inserting to tbl_authors. I insert book details to tbl_books and for authors in tbl_authors. I've tried for each but I can only do it when displaying data when insert I have no idea

   foreach($test as value)
   {
   'author_name' => $value[' no idea what i need to put here']
    }

  public function insertbooks()
            {
              $data=array(
            'book_id'         =>        $this->db->insert_id(),
            'book_title'    =>      $this->input->post('booktitle',true),
            'section_id'      =>        $this->input->post('section',true),
            /*'book_author'   =>        $this->input->post('bookauthor',true),*/
            'book_serial'   =>    $this->input->post('serial',true),
            'book_qty'      =>      $this->input->post('bookqty',true),
            );
            $sql1  = $this->db->insert('tbl_books',$data);
            $author_name=implode(',', $this->input->post('bookauthor',true));
            $data2=array(
            'book_id'       =>    $this->db->insert_id(),
            'author_name'   =>    $author_name
            );
            $sql2  = $this->db->insert('tbl_authors',$data2);
            }