Suddenly, here in the second half of 2009, it seems as though e-readers -- those portable digital devices that can hold thousands of books and use easy-on-the-eyes E-ink -- are finally going mainstream. Yesterday, Barnes & Noble threw its own hat into the ring by unveiling the Nook. This 6-inch digital reader boasts: a built-in, full-color, navigational touchscreen at the bottom; 3G and Wi-Fi connectivity for wireless, straight-to-device book downloading; and innovative ability to share entire books with other cell-phone-, computer-, or e-reader-wielding friends. The Nook joins the iRex DR 800SG and the Plastic Logic Que -- two other Barnes & Noble-compatible e-readers that will launch before the end of the year. (For a list of other new e-readers that have either recently or are soon to hit the market, check out this roundup.)

Like Spring Design's Alex, the Nook is among the first batch of e-readers to run on Android, Google's mobile phone operating system (OS). At launch, at least, this little factoid doesn't seem to really mean too much to the average consumer. Presumably, the implementation of the open-source Android means that innovative apps will eventually be written for the Nook.

In case you've been living under a rock and still don't know what an e-reader really looks like, the Nook will be promoted in big kiosks and sold in all of Barnes & Noble's brick-and-mortar stores (700 retail stores, and 600 college stores, in the U.S.). That being the case, it's sure to get a lot of hands-on when it hits store shelves in late November. (No specific release date was given at the launch yesterday.) In-store perks for owners of the Nook (and other Barnes-&-Noble-compatible e-readers) include: special coupons; the ability to preview entire books; and exclusive content whenever you're connected to the in-store Wi-Fi. Other than those in-store bonuses, though, we don't see much added value in the Wi-Fi, since e-books only take seconds to download via 3G (which, by the way, will be provided by AT&T on the Nook).

The Nook's other unique new feature allows you to share books with other folks. Essentially, this lets you lend out one of your digital books to anyone else with a mobile phone, computer (PC and Mac), or Barnes-and-Noble-compatible e-reader for up to two weeks. Unlike what is too often the case with actual, paper books, you'll actually get these borrowed books back, since they technically never leave your library. You'll need to download special apps on your phone or computer to receive and read these loaners, of course. Unfortunately, specifics were not mentioned as to whether the iPhone, or Google phone, or other phone would be getting this functionality first.

As with the Kindle and other e-readers, the Nook has page-turn buttons on the side, a built-in dictionary, adjustable text size, and 2 gigabytes of internal memory, which allow it to hold about 1,500 books. With a Micro SD memory card, the memory can be expanded by 16 gigabytes, allowing the device to hold as many as 17,000 books. More than one million titles will be available when the Nook launches, 500,000 of which will be free. Most new e-books will cost $9.99 -- same as with Amazon's Kindle -- and newspapers and magazines will also be available (using a format that offers an exact zoomable replica of newspaper pages).

Also soon to be on display are numerous custom cases, designed by fashion names like Kate Spade, Jack Spade, and Elie Tahari, to name a few.

Our objections to the goofy and potentially glitchy and battery-hogging LCD screen aside, the Nook seems a worthy contender to the Kindle, particularly since it'll be part of a much more inclusive eco-system than the Kindle is. (There are simply more readers that will be Barnes-&-Noble compatible than Amazon-compatible.) In addition, Barnes & Noble mentioned the possibility of bundling actual paper books with e-book editions, which we think is a nice compromise for those of us that still like to read actual books on occasion. And, if the number of print publishing industry suits in attendance at the New York press conference yesterday is any indication, this is one device that old media is firmly behind. Plus, this is just the first of many new Nooks to come. Let the e-reader battle begin! (For more pictures and coverage of the Nook, check out Engadget's coverage here.)