Now, you will get a few authors who genuinely say 'I don't care about sales. It's all about the art/achievement/fulfilling a lifelong goal.' But for most of us sales matter - and the more, the merrier. It can come as quite a surprise to a new author that sales are really quite opaque. Mostly publishers will only update you on sales in their twice-a-year (or even annual) royalty statement. Which is sent out typically four months after the end of the previous sales period. That's a long time to wait to find out how your baby is doing.
Clearly the publishers have access to much better information, but rarely do they give access to authors. The only exception I know of is Penguin Random House, which has an 'author portal' website which gives figures for both 'shipped' and 'sold' on a weekly basis, up to last week. (The difference between the two figures is a scary one, bearing in mind the book trade has the archaic system that stores take their books on a sale or return basis.) If other publishers have similar systems for their authors, please let me know.
Otherwise authors really only have one place to turn. Amazon. The behemoth of books has a magic number on the page of every book that has sold at least one copy called its 'bestsellers rank'. In one sense this is quite simple. A big number (potentially in the millions) means it's not selling much at all. Numbers get smaller when more are selling. So, for instance, if I nip to the page of a book I liked very much when I reviewed it, Lee Smolin's Time Reborn, which has been around since 2013, I see this:
The book's 'Bestsellers Rank' (previously called sales rank) is perfectly respectable 16,092. (You will also note that it is number 48 in Astronomy, which is odd (if typical) as it's not an astronomy book, but that's more use for boasting than monitoring sales.) And what an author can do is keep an eye on that bestsellers rank and see how it goes up and down to get a feel for sales. There's even an impressive free (if slightly fiddly) website called NovelRank, that monitors the number for you.
So, I suspect a number of authors felt the bottom had dropped out of their world when a Huff Post blog post announced that 'Your Amazon ranking has nothing to do with sales.' This post by Brooke Warner claimed that 'all your ranking means is that people are looking at your page.' This would be very depressing if true. But is it true? Warner gives no source for her assertion, although in answers to comments she refers to 'my Amazon source', so presumably has a contact who works there - but that doesn't mean she is right.
While no one can say definitively what goes on in Amazon's no doubt byzantine algorithm (or 'logarithm' as Ms Warner engagingly called it until she was corrected) there is strong evidence that it is primarily sales based. First, Amazon actually says it is. They say here, referring to the sub-categories, but then bringing in the main rank: 'As with the main Amazon Best Sellers list, these category rankings are based on Amazon.com sales and are updated hourly.' That's a pretty straight answer. And NovelRank's creator, Mario Lurig, who told me that Warner was 'wrong. Flat out.' has a useful page pulling apart various myths about the ranking - his conclusions may be based on small samples sometimes, but everything he says bears out the experience of regular sales watchers.
It's quite interesting to read the comments of Warner's post, as she becomes increasingly defensive, falling back on the statement that what she really meant was that the rank shows how well a book is selling 'in relation' to other titles. (And says she'll get Huff Post to correct her main text, which may have happened by the time you read this.) However, sadly, this just shows once more Warner's limited grasp of anything vaguely mathematical. Of course a ranking is relative to other titles - that's why it's a ranking, rather than an absolute sales figure. But it still should go up when you make sales, and then decay until your next sale.
The only proviso to this, which Lurig makes clear on his site, is that if you are lucky enough to get a ranking in 3 figures or lower, it won't go up for each sale, because at that kind of level, books are selling fast enough to have more than one per hour, and selling 20 in that hour, say, won't be much different in rank change from selling 21.
So, conclusion? Amazon Bestsellers rank is still the best guide authors have to sales, and when it goes up, you can be pretty sure you have made a sale. Here's my best ranking outside of Kindle (where there are fewer titles) to date. We can always hope for smaller...