A few weeks ago I discovered that you can go to Apple’s App Store and get (for free) Mesillas Yesharim for the iPhone, iTouch, and iPad.
The Mesillas Yesharim app is pretty straightforward, letting you chose either Hebrew or English. The English translation is fairly basic. Of course, if you really want to learn the sefer in English, I strongly suggest finding a copy of R Yaakov Feldman’s excellent translation and commentary (I recently, after searching for a few years, found a copy at a reasonable price and it’s like I’m learning the sefer for the first time again). A full review of this app is available at jewishiphonecommunity.org.
I also, using the built in pdf reader in iBooks, was able to put the original English translation of Rav Hirsch’s The Nineteen Letters on my iTouch. To upload it on your device, simply download it onto your computer, then import the pdf into iTunes (use the “add a file” option) and it will load and default under “books”. Then just plug in your iToy and drag the “book” to your your decice (the same way you would drag a song, album, playlist, or shiur) and sync.