Authors in general, regardless of how their books are published, tend to be interested, if not obsessed with Amazon sales rankings. Like much of the software used by Amazon, it is proprietary and few outside of the software engineering staff of Amazon can determine how it works. Even harder is to determine in any meaningful way what the sales rankings actually mean.
Brent Sampson in his book Sell Your Book on Amazon notes that Amazon Sales Rankings are not meant to be meaningful, but rather just interesting. I find tracking my books on Amazon interesting, but I want to know what the sales rankings MEAN in terms of book sales!
Morris Rosenthal has a reasonably good explanation of how to interpret Amazon Sales Rankings on his blog about self-publishing. To read the blog post use this link to go directly to the information. fonerbooks.com/surfing.htm
As interesting and helpful as Rosenthal's graph and blog article are, I still wanted something I could check and have an actual number that told me with reasonable accuracy. In searching the internet I came across RankTracer, is a subscription based service that will track sales on Amazon. It provides a range of ways to measure sales ranging from books sold in a day or a week. Graphic charts are available to depict sales activity over a range of dates and time periods. If you are interested in learning more about RankTracer visit the site at: ranktracer.com
RankTracer has been interesting to me and provided useful information concerning marketing efforts. As time passes and I am able to collect more data about my books and their sales, I am confident I will learn even more about RankTracer and the capability the site has to offer.