What does Amazon's sales rank really mean?

I recently received a query from a new author who has a book available on Amazon.com, but rather than having a sales rank on the site, his book is shown with "none" as a sales rank. This is an interesting result, but rather than focus just on that, I write about the bigger topic of tracking statistics versus actively promoting a book and talk about some reasons why authors are foolish to pay too much attention to Amazon sales rank anyway.

But don't take my word for it, read what I wrote:

    Figuring out Amazon Sales Rank

If you're an author, I'm particularly interested in your response to my article!