Does anyone know how Amazon.com book sales rankings work?

I asked this question a while back but it never got any responses…I’m hoping new members have joined the Fluther community who might know the answer this time. If a book is, say, # 10,000 in Amazon sales rankings, is there any way to know how many copies it is selling? Is there that much of a difference between # 10,000 and # 100,000? My guess would be that the top 1000 books are the ones that are really selling a lot, and the others probably are selling between 1–100 copies a week. What is considered a respectable number of copies for a book to sell, anyway? Does anyone have more concrete information?