My favorite iPhone app while traveling has been Amazon’s Kindle for iPhone. The initial transfer of the books doesn’t take too long to download over a decent connection given their small size. Amazon also offers free samples of most titles. The app doesn’t require connectivity, has a great notation feature set, and stores an unlimited number of titles given the storage of your device. I’ll be the first to laud the superior format of the traditional book, but it’s tough to beat the convenience of having the entire Kindle library at your disposal while you’re out of range of a good bookstore with English titles.