Backbone Problem

<!doctype html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Testing Backbone</title>
    <link href='http://fonts.googleapis.com/css?family=Open+Sans' rel='stylesheet' type='text/css'>
    <link rel="stylesheet" href="vendor/bootstrap/dist/css/bootstrap.min.css">
    <link rel="stylesheet" href="css/main.css">

    <style>
        body {
            font-family: 'Open Sans', Arial;
        }
    </style>
</head>
<body>
<!-- Begin page content -->
<div class="container">
    <div class="page-header">
        <h1>Books</h1>
    </div>

    <div id="app"></div>

    <!-- Books list template -->
    <script type="text/template" id="books-main-list">
    <div class="row">
        <% _.each(books, function(book) { %>
        <div class="col-lg-6">
            <h4><%= book.title %></h4>
            <p align="center"><img src="data/img/<%= book.thumbnail %>" class="thumbnail" />Donec id elit non mi porta gravida at eget metus. Maecenas faucibus mollis interdum.</p>
        </div>
        <% }); %>
    </div>
    </script>
</div>

<div class="footer">
    <div class="container">
        <p class="text-muted">(C) 2014 Sergio Milardovich.</p>
    </div>
</div>

<script type="text/javascript" src="vendor/jquery/dist/jquery.min.js"></script>
<script type="text/javascript" src="vendor/bootstrap/dist/js/bootstrap.min.js"></script>
<script type="text/javascript" src="vendor/underscore/underscore.js"></script>
<script type="text/javascript" src="vendor/backbone/backbone.js"></script>

<script type="text/javascript">
    var Book = Backbone.Model.extend({});
    var BooksCollection = Backbone.Collection.extend({
        model: Book,
        url: 'data/db.json'
    });
    var books = new BooksCollection();

    var BookListView = Backbone.View.extend({
        el: '#app',

        initialize:function(){

            books.fetch();
        },
        render: function(books) {
            var view = new ShowBooksView({books: books });
            $('.libros').append(view.render().$el)
        }
    });

    var bookList = new BookListView();
</script>
</body>
</html>