The official page (at Amazon) indicates that their sales rank is an account of how frequently a book is selling. My friend and colleague Hugh Longworth has been following the rise and fall of the sales rank on both my book, and his own on steam trains. Due to the niche nature of my book, I've sold well, but never in concentrated bursts to get the sales rank low enough to be important. However, on Friday 1st July (at 14:45) my sales rank hit 4,275, as opposed to the ~60,000 is usually is.
Now if the figures that Morris Rosenthal gives (and I've no reason to doubt him) then that means I've been selling about 30 copies a week. This, however, must less impressive!