From the article intro: "If you are anything like other ebook authors I know, then you probably spend a fair amount of time checking your ebooks’ sales figures and rankings.
The reports offered at some major ebook stores, however, tend to be quite basic. Fortunately there are a number of alternative tracking services available.
Some of the following services also attempt to estimate sales but remember these are only estimates. Unless the source is directly from the ebook store, e.g. Amazon, then this will probably only a “best guess” of expected sales based on a particular rank.
With tools like these you can also keep track of how your competitors’ ebooks are doing and even get some ideas for your next book by researching the performance of particular titles."