How do I show information of a book with ajax

<pre><?php include('header.html') ?>
<br>
<div class="container">
    <div class="card">
        <div class="card-header">Search Specific Book Data</div>
        <div class="card-body">
            <select name="books" id="books" class="form-control" onchange="showBooks(this.value)">
                <option value="">--Select a Book--</option>
                <?php
                require_once('db_login.php');
                //Asign a query
                $query = " SELECT * FROM books ORDER BY isbn ";
                $result = $db->query($query);
                if (!$result) {
                    die("Could not query the database: <br />" . $db->error);
                }
                // Fetch and display the results
                while ($row = $result->fetch_object()) {
                    echo '<option value="' . $row->isbn . '">' . $row->title . '</option>';
                }
                $result->free();
                $db->close();
                ?>
            </select>
            <br>
            <div id="detail_books"></div>
        </div>
    </div>
</div>
<?php include('footer.html') ?>
<script src="ajax.js"></script>

function callAjax(url, inner) {
  var xmlhttp = getXMLHTTPRequest();
  xmlhttp.open('GET', url, true);
  xmlhttp.onreadystatechange = function () {
    document.getElementById(inner).innerHTML = '<img src="images/ajax_loader.png"/>';
    if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
      document.getElementById(inner).innerHTML = xmlhttp.responseText;
    }
    return false;
  };
  xmlhttp.send(null);
}

function showBooks(isbn) {
  var inner = 'detail_books';
  var url = 'get_books.php?id=' + isbn;
  if (isbn == '') {
    document.getElementById(inner).innerHTML = '';
  } else {
    callAjax(url, inner);
  }
}

<?php
    require_once('db_login.php');
    $isbn = $_GET['isbn'];
    //Asign a query
    $query = " SELECT * FROM books where isbn=".$isbn;
    $result = $db->query( $query );
    if (!$result){
        die ("Could not query the database: <br />". $db->error);
    }
    // Fetch and display the results
    while ($row = $result->fetch_object()){
        echo 'Author: '.$row->author.'<br />';
        echo 'Title: '.$row->title.'<br />';
        echo 'Price: '.$row->price.'<br />';
    }
    $result->free();
    $db->close();
?>