php $_POST ist empty or not defined after send value from javascript ajax

I send a value with js/ajax to the same page
the console.log show the correct value but $_POST is empty or not defined after load page
this is my code what did i miss?

<script>
var xmlHttpObject = false;
if (typeof XMLHttpRequest != 'undefined') {
    xmlHttpObject = new XMLHttpRequest();
}
if (!xmlHttpObject) {
    try { xmlHttpObject = new ActiveXObject("Msxml2.XMLHTTP"); }
    catch(e) {
        try { xmlHttpObject = new ActiveXObject("Microsoft.XMLHTTP");}
        catch(e) { xmlHttpObject = null; }
    }
}

function load_data(){
    if (xmlHttpObject.readyState == 4){
        var GetXON = xmlHttpObject.GetXIN;
        document.getElementById(GetXON).innerHTML = xmlHttpObject.responseText;
    }
}
function get_data(N){
  var N   = N;
  var DATA ='data='+N;
  console.log(DATA);
  xmlHttpObject.open('post','',true);
  xmlHttpObject.GetXIN = 'book';
  xmlHttpObject.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
  xmlHttpObject.onreadystatechange = load_data;
}
</script>

<button onclick="get_data('1')">Send Value</button> 

<div id="book">
<?php echo $_POST['data']; ?>
</div>