if(document.images) {
  var seiten = 4;
  Bild = new Array();
  for(i = 0; i < seiten; i++) {
    Bild[i] = new Image();
    Bild[i].src = "images/buch" + i + ".gif";
  }
}

function img_swap(i) {
  if(document.images) {
    forward = document.getElementById("forward");
    back = document.getElementById("back");
    document.images.buch.width = (i == 0) ? 286 : 565;
    document.images.buch.src = Bild[i].src;
    forward.coords = "500,364,544,380";
    forward.href = "buch.php?seite=" + (i+1);
    forward.onclick = new Function("return img_swap(" + (i+1) + ")");
    back.coords = "18,364,62,380";
    back.href = "buch.php?seite=" + (i-1);
    back.onclick = new Function("return img_swap(" + (i-1) + ")");
    if(i == 0) {
      forward.coords = "0,0,286,400";
      back.coords = "0,0,0,0";
    }
    else if(i == seiten - 1) forward.coords = "0,0,0,0";
    return false;
  }
}