I've always been a fan of ebooks, and have read them for years on Palm devices with software from eReader.com.
I wrote a short article a year ago about reading ebooks on the iPhone, and another one last September on using Books.app, but the conclusion in each of those cases is that the iPhone would be a great platform for ebooks, but the software wasn't there yet.
I'm really glad to say that the software is "there" now with the release of several software packages on the iPhone. In particular I've been using eReader, and Stanza for reading on my iPhone, and have been very pleased with both.
In each case the app provides a full screen text view, with a tap or slide motion to turn the pages. Both are extremely easy to use. eReader works with all of my books from eReader.com, and allows me to download books that I have purchased in the past. Stanza will read several formats, but it's biggest use for me is a library of free out of copyright texts that I would otherwise pay for at eReader.
Of the two I would say that Stanza has the best UI, but eReader has the best book selection (by virtue of their online store). They are both free in the iTunes store, so give them a try and let me know which you like best.