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 Barnes & Noble (NYSE: BKS) sank 15% in intraday trading Friday on news that Liberty Media will make a $204 million strategic investment in the beleaguered book retailer.

So what: That kind of commitment would usually be a huge positive for shareholders, but it also means that Liberty's previous $17-per-share takeover bid is now off the table. Of course, given that rumors about Liberty's change of heart were already floating around (the shares are now down 40% in August alone), today's news isn't that big of a surprise to Mr. Market.

Now what: I wouldn't be so quick to pounce on this plunge. Without the glowing prospects of a buyout, buying into a company that's being punished by discounter Wal-Mart (NYSE: WMT) on the bricks-and-mortar side, as well as Apple (Nasdaq: AAPL) and (Nasdaq: AMZN) in the digital space, isn't particularly appealing. Liberty's investment will help, but it seems too little too late.

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