Traitement côté serveur DataTables dans CodeIgniter mais les données ne sont pas affichées dans le tableau

Je peux imprimer les données mais les données ne sont pas affichées dans le tableau.
J'essaie d'utiliser Datatable pour afficher les données.

Voici les messages d'erreur:

Avertissement DataTables: table id = example - Réponse JSON non valide. Pour plus d'informations sur cette erreur, veuillez consulter http://datatables.net/tn/1 p>

modèle

<div class="container">
    <h1>Server Side Process Datatable</h1>
    <table id="example" class="display" cellspacing="0" width="100%">
        <thead>
        <tr>
            <th>ID</th>
            <th>Name</th>

        </tr>
        </thead>
        <tfoot>
        <tr>
            <th>ID</th>
            <th>Name</th>

        </tr>
        </tfoot>
    </table>

    <!--create modal dialog for display detail info for edit on button cell click-->
    <div class="modal fade" id="myModal" role="dialog">
        <div class="modal-dialog">
            <div id="content-data"></div>
        </div>
    </div>
</div>


<script>
    $(document).ready(function(){
        var dataTable=$('#example').DataTable({
            "processing": true,
            "serverSide":true,
            "ajax":{
             "url": "<?php echo base_url('users/Book') ?>",
         "dataType": "json",
         "type": "POST",
         "data":{  '<?php echo $this->security->get_csrf_token_name(); ?>' : '<?php echo $this->security->get_csrf_hash(); ?>' }
                       },
               "columns": [
              { "data": "book_id" },
              { "data": "book_title" },

           ]     
        });
    });
</script>

controller

function Book()
{
    $columns = array( 
                        0 =>'id', 
                        1 =>'title',

                    );

    $limit = $this->input->post('length');
    $start = $this->input->post('start');

    $columns=$this->input->post('order')[0]['column'];
    $order = $columns;
    $dir = $this->input->post('order')[0]['dir'];

    $totalData = $this->user_model->allposts_count();

    $totalFiltered = $totalData; 

    if(empty($this->input->post('search')['value']))
    {            
        $posts = $this->user_model->allposts($limit,$start,$order,$dir);
    }
    else {
        $search = $this->input->post('search')['value']; 

        $posts =  $this->user_model->posts_search($limit,$start,$search,$order,$dir);

        $totalFiltered = $this->user_model->posts_search_count($search);
    }

    $data = array();
    if(!empty($posts))
    {
        foreach ($posts as $post)
        {
            $nestedData['id'] = $post->book_id;
            $nestedData['title'] = $post->book_title;

            $data[] = $nestedData;
        }
    }

    $json_data = array(
                "draw"            => intval($this->input->post('draw')),  
                "recordsTotal"    => intval($totalData),  
                "recordsFiltered" => intval($totalFiltered), 
                "data"            => $data   
                );

    $json = json_encode($json_data); 
    echo $json;
    $this->load->view('templates/header');
    $this->load->view('users/Book',$json);
    $this->load->view('templates/footer');
}

view

function allposts_count()
{   
    $query = $this
            ->db
            ->get('books');

    return $query->num_rows();  

}

function allposts($limit,$start,$col,$dir)
{   
    $query = $this
            ->db
            ->limit($limit,$start)
            ->order_by($col,$dir)
            ->get('books');

    if($query->num_rows()>0)
    {
        return $query->result(); 
    }
    else
    {
        return null;
    }

}