Amazon sales rankings — the addiction of just about every writer out there, whether they admit it or not — are “the poor man’s Nielsen BookScan,” says Steve Weber in a report at Weberbooks.com. As he notes, Amazon’s numbers “reveal the rate at which a title sells relative to competitors. But if you want real, hard numbers — how many copies sold and where people bought them, you need BookScan.”

That’s because Bookscan’s numbers are reports that come directly from cash registers — the company gets readings from Barnes & Noble and Borders and even the virtual cash registers of Amazon and B&N.com, not to mention lots of other book retailers. As a result, Bookscan sales numbers have become extremely powerful — lousy Bookscan numbers can ruin your career (as per this blog post by Alan Rinzler that Weber helpfully points to).

But are figures from Bookscan — which charges subscribers about $5,000 per year — so accurate as to deserve such power? Big-box retailers such as Wal-Mart and Sam’s Club don’t report to Bookscan, nor do many independent bookstores. Weber says Bookscan covers 75% of total retail, but that’s a high estimate on a hotly disputed number. What’s more, it’s a number that shifts per book — actual sales for a book of translated fiction, for example, that isn’t going to sell in the chains like it’s going to sell in the indies, is going to be less accurately reflected in Bookscan that a more mainstream commercial title. Likewise, a huge pop hit that’s going to sell big in Wal-Mart is going to be underrepresented in Bookscan figures, too.

And now, says Weber, there may be “an even bigger leak in the hose: eBooks.” Bookscan hasn’t been tracking electronic sales, and with the explosion of new devices to read ebooks, particularly the Kindle and the iPhone, and with Jeff Bezos claiming (not that anyone believes him) that 35% of Amazon’s book sales are for Kindle ebooks, that’s looking like a mistake.

But one that perhaps can’t be rectified. For one thing, there’s the fact that always-secretive Amazon, surely selling more ebooks than anyone, probably doesn’t want anyone to know what its actual sales are. What’s more, says Weber, “I wouldn’t be surprised if Amazon launched a BookScan rival.” Then there’s the fact that ebooks have no “ industry-standard identifier,” such as an ISBN, allowing sales to be tracked. (Not that they couldn’t have one — Amazon, for example, gives its ebooks its own Amazon Standard Identification Number.

But beyond the fact that it’s making Bookscan seem all the more unreliable as a measure of actual book sales, while Amazon once again looks like an 800 pound gorilla in yet another realm of the book business, what does it mean?

Says Weber, “If I were working at BookScan or at a company who provides services for self-publishing authors, I’d be dusting off my resume. Or maybe uploading it to the Kindle.”