Books.app is a simple eBook reader for the iPhone. It reads HTML and text files stored in your Media/EBooks folder, and is smart enough to enter subdirectories, if, for instance, you've broken a book down by chapters.
Eventually, this project will include a simple method of syncing eBooks to your iPhone. At the moment, that's handled by iPHUC and a shell script (eventually).
Books.app is recommended for use with Project Gutenberg texts, in conjunction with GutenMark, a fantastic PG markup tool by Ronald Burkey, which makes pretty HTML out of Gutenberg .txt files, and splits them by chapter using a second tool.