Once you have a book published, it's very tempting (if extremely tedious) to keep checking the sales rank on Amazon. Sales rank is a single number that (in theory) ranks your book for number of sales against all other books on the site. So it can go from 1 ('You may soon be a millionaire') to n million ('Don't give up your day job.') You can't read too much into this figure, but clearly every time it bobs down to a significantly lower value, you have made a sale.
It really is quite possible to drive yourself mad, checking these numbers. But now there's a website to help drive you even further towards being a raving wreck. Novel Rank monitors the page rank of your book, and converts its movement into sales numbers. It's a little bit fiddly to set up. You have to draw a book to its attention by putting in the Amazon URL for it. You can then pull up a page for that book whenever you like, showing rank, sales and sales history since you added it on Amazon.com/co.uk/.ca (optionally you can add in the non-English language Amazons).
Alternatively, if you have several books published, you can pull them together on a single page if you set up an account (it's free and trivial to do). You only see the results on your account page for one Amazon at a time and there's no graph, but if you let your mouse hover over a book it pops up the rank and sales numbers on the other Amazons.
It's truly wondrous/horrifying (depending on how well your book is selling). Just don't say I didn't warn you. Visit this site and you might never be able to stop going back.
Thanks (I think) to Jessica Ruston for bringing this site to my attention.