Whether it's Big or Small, Data is the key to making decisions. That's why Amazon launched its ebook pricing tool last year, and that's why Wesley Atkins developed KindleSpy (warning: link leads to site with auto-play video).
As you can see in the demo video, KindleSpy is designed to scrape a category in the Kindle Store and reveal a few key details including estimated earnings, popular keywords among the best selling titles, and so on. An author can use that info to choose which categories to focus their attention on, and also use it to pick a title with all the right keywords.
While this sounds like a nifty tool, it's not without its detractors. I heard of KindleSpy via The Passive Voice. It's been around for close to a year, and a number of commenters over at TPV have tried it.
Several have pointed out that buying KindleSpy invites a flood of spam from the developer, and one went on to offer a lukewarm recommendation:
I’ve been using it for a few months, and have experienced good and bad with it. I do think it can help you identify keywords that will work better for your book, although I suspect the results will benefit non-fiction authors more than fiction.
Another commenter expressed frustration with the tool:
The point of it is to find categories/keywords for you where your book has a fairly good chance of floating to the top. This… would be great if the categories/keywords that apply to your book aren’t already completely saturated. :,
Speaking of competitors, there's also KD Researcher (warning: auto-play video). I've only just heard of this app (all 3 tools are new to me), but I did find a comparison review for KindleSpy and KD Researcher. The review is 8 months old but should still prove useful. For example, it told me that KD Researcher works with several bookstores, not just the Kindle Store.
I wouldn't touch any of the three tools; the auto-play videos and sales tactics have convinced me that these guys aren't too be trusted.
But these tools could be useful to you. They don't appear to be in widespread use yet, and the author who knows how to wield them effectively will have a competitive advantage.