Now we know why the current Kindle suddenly became unavailable earlier this week, with no date listed for stock to be replenished: It’s because Amazon is launching a new-model Kindle, one that’s smaller, lighter, faster, and easier to read.

The new Kindle hangs on to everything we’ve come to love from the current model: Same 6-inch screen and same 3G wireless connection for downloading books in the U.S. or internationally. It’s still $189 too.

But virtually everything under the hood has been upgraded: The new model is 21 percent smaller and 15 percent lighter, capacity has been doubled to carry 3,500 books, and Wi-Fi has been added for when you’re out of range or away from a 3G network. And the screen has now been upgraded as well, using the same higher-contrast technology that the upgraded Kindle DX offers. Page turns are 20 percent faster, and it’s available in the classic white or graphite color options.

Amazon is also offering another version of the Kindle, called the Kindle Wi-Fi. This model, an amazing bargain at just $139, offers the same features as this Kindle but strips out the 3G connection altogether. Instead, you have to do all your book downloads via Wi-Fi, a trade-off that many who don’t read far from home may find entirely worthwhile.

With 630,000 books available on the Kindle Store (plus 1.8 million free, pre-1923 novels), the rest of the Kindle world remains about the same. But these new offerings show just how aggressive Amazon is about owning the e-book-reader space. What more can the company offer to prove its commitment to e-books? Aside from giving Kindles away for free and figuring out how to turn these displays into color, I mean.