About once a day, I check how our book is doing on Amazon.com. Ever since our second edition came out in December 2006, it's been hovering around 2500-4000 in terms of overall sales rank. Checking this morning, our book is at #388. Assuming that Amazon's sales follow a Zipf curve (or is it power law or Pareto? I can never remember), this means a heavy increase in sales.
The problem, though, is that Amazon doesn't reveal what their rankings actually mean, and I only see how many books we sell in 6-month periods, so it's hard to say more with any precision.