Since then a lot has changed:
nook for iPhone app from Barnes & Noble. This app allows owners of the Nook, B&N's rival to Amazon's Kindle — as well as non-owners of Nooks — to read Nook e-books on their iPhones.
iBooks for iPhone. It's the iPhone app you need to use to buy e-books from Apple's iBookstore, an arm of its iTunes Store.
Kobo is another new e-reader app. It lets the iPhone read e-books meant for the new $149.99 Kobo eReader device being sold at Borders stores in the U.S. That device supports open ePUB, open PDF, and Adobe DRM e-book formats.
Borders iPhone app is, as far as I can tell, exactly like the Kobo app except in minor details.
All of these apps are available (often in separate versions for the two devices) for the new Apple iPad, not just the iPhone.