I've never really put too much attention into the Amazon sales rank of a book until my first book got published and finally showed up on Amazon (this was another issue of its own... See HERE). But now that I've started paying attention to the values, I have noticed some very odd behaviors.
Currently it seems like the rank for my book "Shaders for Game Programmers and Artists" is about 3,300. However even if this seems good, it still makes me wonder considering the history of my ranking since the book was listed a about two months ago.
Obviously, my book started in the 2,000,000 range initially. Which is very likely since the book was not listed at all meaning it most likely had no pre-orders. Over the next 1-2 weeks the score slowly crept up to the 1,000,000 range.
Then something weird happened. All of a sudden i went from a rank in the million range to 8,000!?!?! Not believing it, i tried refreshing the page and that's there the weirdness happened. Refreshing the same page several times in the span of a few minutes, I've noticed that my rank was toggling between an 8,000 rank and an 80,000 rank. After keeping an eye on it it eventually stuck to something in the high 80,000's. So i guessed the 8,000 was some bug or fluke on Amazon's end.
Over the next few weeks, my score crept up some more, back into the 200,000-300,000 range. But i guess as people actually started noticing the book and awareness increased the trend reversed and went back all the way down to a 40,000 number, which held for a good 1-2 weeks.
But then the trend started reverting again and my rank slowly started creeping up until it reached the upper 90,000's. Then a little over a week ago, my rank jumped from around 100,000 to about 800,000. But then all of a sudden, today the number went from 790,000 back down to 6,500 and now down to 3,200. (actually, just rechecked and I'm now at 2,600).
Getting an email from an old friend of mine might explain the sudden rank improvement of today. Seems like Amazon sent their weekly email featuring my book this week (as a new release?!?! Well it's been out for four months!!). But it does seems like it has generated a new influx of sales. :)
Now, I've read an interesting analysis of how the Amazon ranking system works by Fosner Books. Which might explain some of the fluctuations but still as an author I would rather have a simpler number which gives a better indication of how a book is really doing.
For example, why not simple take the total sales, divide them by the number of days the book has been listed and determine a sales/day number which can then be uses as a basis to rank all the Amazon books?