You can store a book on your iPhone -- find a book you'd like to read, then click the "Bookmarklet" link! Open the URL, save to your iPhone "Home Screen", and you'll have something ready to read, even without a net connection, thanks to the wonders of data:URL.
This appears to work only with firmware 1.1.4 - firmware 1.1.3 will fail!
Download and install Installer.app on your iPhone -- it's a simple process (a very thorough walk-through is available on Gizmodo.) Afterwards you'll be able to install applications like Books.app with just a few taps. Books.app also has a straightforward script for copying HTML files to your iPhone (try an "iPhone Books.app" file from manybooks!) available at http://code.google.com/p/iphoneebooks/wiki/InstallingEBooks.
You can also install books directly to your iPhone using Installer.app by creating a bookshelf at manybooks.net and adding it as a repository source to Installer.app. When you update your shelf at manybooks, refresh the source list for your iPhone's Installer.app - then install the books from the "ManyBooks" category.
There's a good walkthrough of this process at ModMyiFone.com.
Sources for the most recent additions to each Category are also available in the plist directory.
Browse manybooks.net or mnybks.net with your iPhone. Specialized CSS is utilized in both cases, but mnybks.net is also stripped down and quick. When browsed with an iPhone the full manybooks.net site has links to HTML and PDF downloads on mnybks.net.
Some other folks are working on solutions that may work for iPhone users - the most notable ones are: