You can install the Amazon Kindle app for your iPhone. It does not allow you to purchase eBooks. You do not have to convert.
As to converting to ePub, you'll need to read
this thread. You will also need to install Calibre. See the Calibre forum if you need any help there. Also you'll need to install the DeDRM plugin for Calibre that you can get at
Apprentice Alf's blog.