$(function() {
  $(".next").stop().click(function() {
    nextBook();
  });
  $(".previous").stop().click(function() {
    previousBook();
  });
});

function nextBook() {
  var $active = $("#book img.active");
  var $next =  $active.next().length ? $active.next() : $("#book img:first");
  $active.addClass("last-active");
  $next.css({opacity: 0.0}).addClass("active").animate({opacity: 1.0}, 600, function() {
    $active.removeClass("active last-active");
  });
}

function previousBook() {
  var $active = $("#book img.active");
  if (!$("#book img:first").hasClass("active"))
  {
    $previous = $active.prev();
    $active.addClass("last-active");
    $previous.css({opacity: 0.0}).addClass("active").animate({opacity: 1.0}, 600, function() {
      $active.removeClass("active last-active");
    });
  }
}