The Amazon ranking for your title gives you an idea of how popular your book is in the past hour. A rank of 1 = the best, hottest selling book. The challenge with tracking your rank is that it changes over the day and doesn’t really tell you overall how you’re doing, just how you’re doing that hour.
Admittedly you can make assumptions about how well your book is doing by looking at different ranges, 1-1000 being damn good.
You can track your Amazon ranking by RSS or Twitter:
Booklert from MCQN.