Books are having their iPod moment this holiday season. But buyer beware: It could also turn out to be an eight-track moment.

While e-reading devices were once considered a hobby for early adopters, Justin Timberlake is now pitching one on prime-time TV commercials for Sony Corp. Meanwhile, Amazon.com Inc.'s Kindle e-reading device has become its top-selling product of any kind. Forrester Research estimates 900,000 e-readers will sell in the U.S. in November and December.

But...