Rankingly Obsessed

Amazon has sold more than 7.5 million unique titles at this point, and rankings indicate a title must sell at least one copy a year to remain above a rank of two million - Morris Rosenthal, Foner Books 

Getting your book published is a exciting, awesome journey, but once it hits the shelves, you might think that writers relax and wait for the royalty checks to arrive in the mail... alas, it's not true. I'd say the majority of the writers I know start obsessing (at least a little) about sales. The first place they look is at their Amazon sales rankings. Your Amazon sales rank is a number that says how many other titles sold more than your book. The smaller the number, the better the sales. The number is re-computed daily (for obsessors who need to know)

 For example, a major publisher tracked 25 titles over a six month period, correlating the weekly Amazon sales rank with actual reported sales from Amazon. Here is what they found correlating Amazon Sales Rank with real sales:

Amazon Actual
Sale Rank              Books Sold per week
---------------------     -------------------------------
75-100                  250-275/wk
100-200                225-249/wk
200-300                150-200/wk
450-750                75-100/wk
750-3,000             40-75/wk
3,000-9,000          15-20/wk
10,000+                1-5/wk

Source: Rampant Techpress

There are fluctuations in Amazon sales rank when the book is first released - When the initial backorder is filled, the sales rank plummets (sometimes below 1,000) for a brief period. Thee there may be a large drops in rank when there is a bulk order. Not all books are treated equally -- the top 1,000 are recalculated hourly, the next chunk (up to 100,000 (estimated)) weekly, while the rest, monthly. 

Also available to obsessed writers are helpful Internet sites which track you sales for you, like Novelrank.com. NovelRank uses Amazon sales data, and is a free website for authors to track their Amazon Sales Rank on Amazon.com, Amazon.co.uk (United Kingdom), Amazon.ca (Canada), Amazon.fr (France), Amazon.de (Germany), and Amazon.co.jp (Japan).

So, feel free to go forth and obsess, but not too much!