Just how much of a reflection of how well (or maybe not so well) a book is selling IS Amazon's book *ranking?
Amazon rankings are meaningless.
Statistically, they measure your book only in relation to the sale of other books, not how many copies actually sell.
Here's an example: Miss Snark's Guide to Query Letters is published to great acclaim and fanfare. Devoted Snarklings rush to buy it. One thousand copies are sold in one day! Her Amazon ranking shoots up to #1. Great rejoicing at Snark Central. The key piece of information that is not apparent in that statement is that Miss Snark's ranking is #1 because Amazon didn't sell 1000 copies of any other book that day.
Second day: Miss Snark is modestly mute about her fabulous work but Snarklings reading the archives rush to purchase the book on day two. Sales rocket. Two thousand books fly out of the warehouse. Miss Snark’s Amazon ranking plummets to #10 because nine other books sold more than two thousand copies that day.
You see the madness: you can sell more on day two AND have your ranking drop.
Authors can't resist looking at their rankings and grumbling mightily if they perceive themselves as "low" on the list. There is no way to talk them out of looking at those rankings. I've tried till I'm blue in the face--and blue is NOT a good color on Miss Snark.
If you can possibly avoid getting caught up in the rankings madness, your peace of mind, not to mention your family and everyone who knows you will be grateful.