The simple answer is that it means different things to different people, but I’ll elaborate.
If you’re an Amazon.com customer looking at the “Product Details” for a specific book, you might interpret Amazon.com’s sales rank as a measure of that book’s popularity. The lower the Amazon.com sales rank number, the more popular the book will be as determined by the number of recent sales for that title through the Amazon.com marketplace. This is true in general, but if you really want to know the popularity of a specific title, you should also consider the publication date and how long it has been trading on Amazon.com. For example, a book with an Amazon.com sales ranking of 100,000 that has only been released and traded on Amazon.com for a few days may be very popular in the near future and make it into the top ten. That it’s only ranked 100,000 right now is a bit misleading because the time period over which this rank was determined isn’t long enough to gauge true popularity.
For a bookseller, Amazon.com’s sales rank often correlates with how quickly a particular title can sell. A book with a very low number Amazon.com rank can be expected to sell within minutes or hours of listing, and a book with a sales rank of 4,000,000 can take several years to sell. Booksellers frequently use the analogy when looking for books to add to their inventory. If a bookseller finds a book that has an online value of $10, an Amazon.com sales rank of 2,105,878, and is faced with buying it for $4, they are likely to pass, but if the sales rank were 45,017, it might suddenly be a book worth adding to your inventory.
When looking for books, I look at Amazon.com’s sales rank for higher-priced books because I don’t want to pile a lot of firewood on my shelves, but it’s certainly not the measurement I use for most book purchases. The scientist in me wanted to know more, so I compiled Amazon.com ranking data for thousands of books I’ve sold, using Amazon.com’s ranking both at the time I bought them and at the time I bought them. I sold them, respectively. Being an engineer, I had to collect and torture data to see if there was any correlation between Amazon.com sales rank and the time it takes to sell a book. In fact, there is, at least in my experience, and curve fitting the data yielded a correlation coefficient of 0.93. The good fit of the data is particularly evident on a semi-log plot.
So, you see, there is meaning to Amazon.com’s sales rank.
For the book buyer it is a measure of the popularity of a book.
For the bookseller, it’s a measure of how quickly a book can sell and whether or not the book can be just kindling.
For the engineer, it’s just a bunch more data that can be tortured into some sort of confession.
