The eReader app is not a usable option on the iPad. Either (a) you keep it at the original size and it takes a fraction of your screen (roughly iPod Touch-sized), or (b) you blow it up and the text looks terrible. You should not expect that eReader will be updated for the iPad anytime soon - if ever - since it has been purchased by Barnes and Noble. You will not be able to import your books the B&N application.
You will also not be able to import your books to the Kindle application. Even if you were to jailbreak it, placing files in the Kindle's data folder is not guaranteed to make the Kindle application read them.
I recommend using calibre to convert them to ePub. As long as your books are not DRM'd, this will be a very simple process
. Even if they are DRM'd there are ways around this problem if you like cabbages. After converting, you can use any non-DRM ePub reader to read them, including iBooks. I personally am currently using readMe, because of its support for downloading epubs from the browser. (I run calibre server and download them from there.)