Unless I'm missing something, this site lets you track sales of any book on Amazon for a year.
The AW Amazon Store
Buy Books by AWers!
Handy, yes - but not guaranteed to be accurate.
It's a good resource but don't use it over your own KDP info screen.
Achievers strive for excellence. Perfectionists drive themselves to extinction. -- A Grapple A Day
I've never known any trouble that an hour's reading didn't assuage. -- Charles DeSecondat
II 2016: 2017:
I've found it woefully inaccurate for fast-selling titles, too. It'll show the accurate rank at a moment in time, but the sales numbers have never been close on any of my books, regardless of how well they do or do not sell.
"Now, come on, as you guys get older you'll realize people don't mean to be obnoxious, it's just that they're all screwed up inside." -- Joel, MST3K, Gamera
The numbers are close to accurate for my slower-selling titles. But, yeah, it really underestimates on anything selling well.
Still, it's convenient to have Amazon links to all my books with most of the different Amazon sites (.uk, .jp, etc.) all in one place. I check it daily. (And then don't really DO anything with the information, but... I check!)
ETA: I also use it to check on other authors - not necessarily in a competitive way, but if someone's offering lots of advice/proclamations on publishing or book promotion, I'll type their name into Novel Rank and get a rough estimate of where their sales are. Higher than mine? I should consider the advice. Lower than mine? I'll still consider it, but only on its own merit, not as something from 'an authority'.
It regularly undercounts my two-dozen or so monthly sales, but it's fairly good at tracking really low sales.