Bookstore chain Borders announced Wednesday it has opened its own online marketplace called Borders eBooks. And like its competitors, Borders eBooks will vie for market dominance by running on a multitude of devices, including PCs and Macs, Kobo and Sony e-readers, Apple's iPad, iPhone and iPod touch, and soon on Blackberry and Android gadgets.

It's getting harder to keep track of all the ebook stores popping up online. Borders joins Amazon's Kindle Store, Apple's iBookstore, Barnes & Noble's Nook site, and the Sony Reader Store. All told that's five major competitors vying for your e-book buying dollar. Let's hope keep our fingers crossed these e-book marketplaces compete on price - competition is driving e-reader prices down.

Borders has partnered with Kobo, a global e-reading service that will operate the bookseller's online store. In fact, Borders Group is one of Kobo's financial backers. Kobo also makes a no-frills e-reader (eg, no keyboard, touchscreen, search tools, apps and other essentials) that Borders sells for $150, a price that seems high now that the more sophisticated Kindle and Nook cost just a bit more.

Amazon and Barnes & Noble cut their e-reader prices last month, and Sony followed suit yesterday.

Most of Borders ebooks are in the in ePub format, although some titles will have Adobe PDF and mobile-specific versions too. Unlike its leading competitors, specifically Amazon and Apple, Borders' ebookstore won't sell digital magazines or newspapers, at least not right away. Borders' customers probably won't mind, as much of that journalistic content is available for free on the web, at least for now.

Since Borders' store has more than one million titles, there's no shortage of content, even for readers with eclectic tastes. The big question, however, is how Borders will distinguish itself in the ebookstore war. With three fierce competitors, Amazon, Apple and Barnes & Noble, already open for business, Borders will need to stir up trouble to lure visitors to its site.

OK, examples. How about slashing the Kobo e-reader to $99, an online-only price available exclusively at Borders' e-store? Or Borders could one-up Barnes & Noble's LendMe policy, which lets Nook owners lend ebooks for up to 14 days. (Borders' customers can't lend ebooks or give them as gifts to friends.) A third option: Allow Borders' customers to buy the printed and digital edition of a title at the same time. Currently, that's a no-no too.

Granted, publishers may balk at some of these ideas, but the cutthroat competition in the burgeoning ebook market is bound to change some minds.