Google Analytics.
Have been adding each and every hub made to this process and reporting tool every time I have written. Since the middle of May 2010 I have seen a steady increase in my readership yet also in the number of clicks. Both in terms of AdSense and Amazon revenue, these have also increased. As a matter of fact I have seen over the last 4 weeks a four-fold increase in my Amazon click through. Most likely as half of my hubs now (22 out of 43 at the time of this writing) are Amazon specific hubs.
Analytics is a very useful tool, it tells you where your traffic is coming from, how many visitors your getting each day, week, month, how many people are clicking on more than one page and which pages they are starting at and where they are leaving. It also tells you how much AdSense income is coming in on a daily basis. The other cool aspect all these items and much more info, are supplied as graphs, so you can visually see where your going with it all.
If your not using it, your missing out on using a valuable tool. Link supplied.