Ah, the ever popular "Amazon Sales Rank." Often debated, never fully understood. Amazon doesn't say a whole lot about what the ranks mean on their site so we're left to speculate like the number-crunching data addicts that we (or at least some of us) are.
I've seen several good sales rank blog posts over the last few years but this one from the Beneath the Cover blog is probably the most useful, straightforward summary of all. Blogger Bill Stephens also mentions he's going to focus a future post on showing how to calculate total retail sales from the Amazon sales rank...
To be honest, I've seen this tried before with mixed results at best. There's really no tight connection between Amazon sales rank and how many copies of a book have sold at Borders, Barnes & Noble, etc. It's generally safe to say that a highly ranked book on Amazon is also doing well at the brick-and-mortar outlets, but I know of know formula that accurately calculates total retail sales off the Amazon rank.
If you're as curious about Amazon sales rank as I tend to be you should also look at two related services: TitleZ and RankTracer. Also, here's another fascinating look at Amazon sales rank from the folks at Dog Ear Publishing; this article includes some actual sales results from Dog Ear titles.