Well, not actually - but maybe soon.
The gadget site Gearlog just posted an interview with Apple's Greg Joswiak, Vice President of Hardware Product Marketing.
The operative part of the interview is as follows: "I asked him about independent, native software development for the iPhone. He said Apple doesn't oppose native application development, which was new to me. Rather, Apple takes a neutral stance - they're not going to stop anyone from writing apps, and they're not going to maliciously design software updates to break the native apps".
This is the first time, to my knowledge, that Apple has acknowledged that third party applications will be "allowed" on the phone. Looking around for such a stand-alone application for ebooks, I found Books.app. This is an installable application that can read HTML and text files, and , according to the site, can read Project Gutenberg books.
Installing the application isn't easy, and I'm not about to try it and risk my phone, but this is an exciting development that may bode well for the iPhone and e-book community.
By the way, a good listing of stand-alone iPhone applications can be found at the ModifyMyiphone site.