Shows updated number of reviews from amazon in a big ugly div and in the page title. Go to a book page (e.g. https://www.amazon.com/100-Tricks-Appear-Smart-Meetings/dp/1449476058/) and paste this into the JS console.

amazonReviewTracker.js

// Update every 3 seconds, may want to change this.
var updatePeriodMillis = 1000; 
var lastRevs = 0;
var d = document.createElement('div'); 
d.innerHTML = 
    '<div style="position: absolute; left: 150px; top: 250px; background-color: rgb(119,0,0); width: 1200px; height: ' 
  +             '800px; color: #fff; font-family: arial; right: 150px; padding: 20px;" id="reviews">'
  +   ' <div id="numReviews" style="position: relative; font-size:6em; left: 10px; top: 10px;">--</div>'
  +   ' <div style="margin-top:20px; position: relative; font-size:1em; left: 10px; top: 10px;">Last updated</div>'
  +   ' <div id="lastUpdated" style="margin-top:10px; position: relative;font-size:2em; left: 10px;top: 10px;">not updated</div>' 
  +   ' <div style="margin-top:10px; position: relative; font-size:1em; left: 10px; top: 10px;">Last change</div>'
  +   ' <div id="lastChange" style="margin-top:10px; position: relative; font-size:2em; left: 10px;top: 10px;">not changed</div>' 
  + '</div>';
document.body.appendChild(d);
var update = function() {
  var getReviews = function(s) {return parseInt(s.match(/showViewpoints\=1[^>]+>(\d+)<\/a>/)[1]);}; 
  var asin = String(document.location).match(/dp\/(\d+)/)[1];
  var args = 'method=getBookData&asin=' + asin;
  var url = 'https://www.amazon.com/gp/search-inside/service-data';
  var req = new XMLHttpRequest(); 
  req.onreadystatechange = function() {
    if (this.readyState == 4 && this.status == 200) {
      var revs = getReviews(this.responseText);
      console.log(revs); 
      if (revs) {
        document.title = String(revs);
        document.getElementById('numReviews').innerHTML = String(revs);
        document.getElementById('lastUpdated').innerHTML = new Date().toLocaleTimeString();
        if (lastRevs != 0 && lastRevs != revs) {
          document.getElementById('lastChange').innerHTML = new Date().toLocaleTimeString();
        }
        lastRevs = revs;
      }
  }
};  
  req.open('POST', url, true); 
  req.setRequestHeader('Content-type', 'application/x-www-form-urlencoded'); 
  req.send(args);
};
update();
setInterval(update, updatePeriodMillis);