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 book and Nook seller Barnes & Noble (NYSE: BKS) watched investors close the book on its shares today, as they fell as much as 31% in intraday trading after the company lowered its forecast for full-year results.

So what: On the bright side, sales of B&N's Nook reader were up significantly from last year's holiday season. However, those sales look like they'll miss expectations. Worse, investments that B&N has made to help fuel that growth are also fueling a much-larger-than-expected loss for the year.

If you ask B&N management, though, the Nook business is absolutely one worth owning -- perhaps even on its own. In its comments today, B&N noted that it's exploring strategic options for the Nook business, including spinning it off.

Now what: I have a really tough time getting excited about Barnes & Noble as an investment. On one hand, you have a brick-and-mortar bookseller. It's done better on that side than its bankrupt rival Borders, but it's an imperiled business nonetheless. On the other side, there's a promising growth business in the Nook e-readers. That's a more interesting segment, but it's costly, and it's going head-to-head with (Nasdaq: AMZN), a company with a balance sheet that outshines B&N's by an extremely wide margin.

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This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.