The FictionWise link is http://www.ereader.com/iphone/
I can't download it yet, since the 2.0 firmware isn't out for my first gen iPhone, but the App Store is open now in iTunes 7.7, and a quick browse through the entertainment category turned up eReader.
[After you install iTunes 7.7, open preferences, go to the General tab, and check the box to display "Applications". Then you can view Applications...which will be empty, since you don't have any yet, and click on the "Get More Applications" in the lower right corner.]
The reader itself is free. From the description, it appears that you download the books to the iPhone directly within the eReader app...I'm curious how you get books that you buy from someplace other than FictionWise onto the phone.