Lately, I have been hearing this question a lot. Almost as often as "how are you?".
For quite some time, I had no concrete answer. My publisher would tell me actual sale numbers only at the close of the quarter. And even then, I will have no way to know whether the numbers are good or bad. Also, I will have no idea of the trend. Did all the books got sold in first few days? last few days? is the rate improving or what?
I could watch Amazon.com sales rank of my book several times a day, which I did, and get some idea. It is still too much work explaining this to a casual questioner. (Well, most of those who ask this don't expect a serious answer, but some do!)
For those few, I do have some answer now -- thanks to this excellent service. I, and even you, can watch how the Amazon.com sales rank of my book has moved in last few days/weeks/months by watching this graph. It is quite positive so far, as you can see. However, the present trend is no gurantee that it would remain so forever.
Deciphering Amazon.com sales rank and translating that into actual no. of copies sold is a black art and has received a good amont of scrutiny. If you are really interested, read this eye-opener.
You can also compare sales rtank of two books to get an idea of the relative performance. Based on my own observations on a handful of computer books and other online research, I can say that the rank tends to fluctuate widely for newer or less-successful titles. It is quite common for a new title to fluctuate between X and Y where X is less than 10,000 and Y is around 100,000. With time, Y tends to drift lower and come closer to 10,000, at least for moderately successful books. But the author and publisher don't make much money unless the book remains below 10,000 for a sustained period of time.