I think I’ve finally figured out how many Kindles Amazon has sold. It’s been a closely-guarded secret for years — but it’s possible to calculate a good estimate using the new information Amazon released yesterday.

Monday Amazon announced the Kindle 3 had become the best-selling product ever in Amazon history. That’s even more impressive when you consider that the Kindle 3 has been available for just four months. And two weeks ago, Amazon bragged that they’d already sold “millions” of Kindle 3’s in just the first 73 days since September 1. (That’s pretty much the entire life of the Kindle 3, since it was released just four days earlier, on Saturday, August 27.)

Obviously that confirms that Amazon has sold at least two million Kindles. But you’d think they’d find a way to confirm an even higher number — if in fact they’d actually sold more. And then Monday Amazon’s CEO provided another crucial data point for comparision. He announced coyly that Amazon had sold more Kindle 3’s than they’d sold of the final book in the Harry Potter series.

Interestingly, Amazon once claimed that it didn’t earn any money on the sales of that Harry Potter book. Amazon sold Harry Potter and the Deathly Hallows at a 50% discount to attract new customers — for just $7.99 — and three years later, it’s still listed on Amazon at that same low price. That book had been the previous best-selling item in Amazon’s history, but ironically, the popular book about the young sorcerer now holds the key to another grand mystery. If we knew how many copies Amazon sold, we’d also know their secret sales figure for the Kindle 3!

But we do know how many copies Amazon sold — at least, judging by a 2007 press release. Amazon announced they’d sold 2.5 million copies of the Harry Potter book in its first 10 weeks of release, according to The New York Times.. But more than 1.6 million copies were pre-ordered, according to an Amazon press release — making the announcement a full 19 days before the book was released. They’d sell just 900,000 more copies over the next 12 weeks, if you combine the information in the two sales figures.

And meanwhile, the book was selling like magic in offline bookstores around the world. In fact, 15 million copies were sold on its first day, according to Forbes magazine. More than a year later, they announced that 44 million copies had finally been sold. Is it possible to calculate Amazon’s share of those sales, thus revealing their sales figures for the Kindle 3?

Note that the sales figures over a year later were just triple the sales figures from the very first day. Applying the same formula to Amazon, you’d expect Amazon to sell 7.5 million copies of the book by the end of 2008. And there’s another way to guess Amazon’s sales figure, which results in a nearly identical number. A business analyst once reported calculations that Amazon sells 19% of all printed books. Using that as a rough guideline, Amazon would’ve sold 9 million copies of the Harry Potter book by the end of 2008.

Of course, they’ve probably sold even more copies since then — especially since last month saw the opening of a movie based on the book. But what’s interesting is this estimate is pretty close to the leaked sales figures that Bloomberg News reported last week. Amazon was expecting to sale 8 million Kindles in 2010, according to “two people who are aware of the company’s sales projections.”

No matter how you estimate it, you seem to come up with the same number – so I feel confident in saying that Amazon has sold close to 8 million Kindle 3 devices in just the last year. Of course, there’s also lots of people who own a Kindle 2 (and even the original Kindle 1), so the number of Kindles in the world is probably much higher. Still, it’s another milestone along the road to the ebook revolution. And Amazon revealed in another interesting piece of trivia from their Monday press release. Saturday more people purchased more Kindle ebooks than on any other day in history.

Ironically, there was one book they weren’t buying: Harry Potter and the Deathly Hallows, which is still only available in its print edition!