var page_width = 960;
var $pages = $("#pages").animate({height: $('#page_1').css('height')}, 600, 'swing');
$("#nav a").click(function(){
  $this = $(this);
  var page_number = $this.attr('data-page');
  var $page = $('.page[data-page="' + page_number + '"]', $pages);
  $pages.animate({
    left: - page_width * (page_number - 1),
    height: $page.css('height')
  }, 600, 'swing');
  return false;
})