Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Shares of still-standing bricks-and-mortar bookseller Barnes & Noble (NYSE: BKS) had its binding broken by investors today, falling as much as 13% in intraday trading after the big announcement from (Nasdaq: AMZN).

So what: Today Amazon introduced new products in its very-successful line of Kindle e-book readers. There was a redesign of its classic Kindle and the introduction of the Kindle Touch. And, oh, yeah, there's also now a Kindle tablet that can be summoned by uttering the phrase "Kindle Fire." To quote Paris Hilton, "That's hot." Or at least it sure sounds like it.

Why is this bad news for Barnes & Noble? Die-hard fans of also-rans will almost certainly know that the on-the-ground bookseller sells a competing e-book reader, the Nook. Obviously, it's bad news for B&N if Amazon is able to further extend its market-share lead.

Now what: B&N's stock came back from the early drop and finished the day down 6.89%. Frankly, the rebound doesn't surprise me, because it shouldn't be news to anybody that B&N has lost to Amazon already. A recent earthquake in Washington, D.C., reminds us that anything can happen, but I don't think it took this announcement to convince B&N shareholders that the Nook isn't going to slingshot Amazon's Goliath.

Who should be concerned about Amazon's announcement? If you're an Apple (Nasdaq: AAPL) shareholder, you have to be perspiring at least a little -- the Fire ain't an iPad, but with its $199 price tag, there may be a lot of consumers who don't care. And with the recent announcement that Amazon has added more movies to its library, the integration of streaming movies with its new tablet means that Netflix (Nasdaq: NFLX) can't ignore what Jeff Bezos & Co. are up to.

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