Convert NOOK (Barnes & Noble) Book (ePub) for iPad and Kindle

We are nearly done creating a basic epub book for the Barnes & Noble Nook (I believe they have their own proprietary epub format). Creating a Nook book is a walk in the park as they offer very easy to use software and while functionality is limited, it looks great. The book that we have created is very easily editable as we have each page as a separate .PSD file with the text as a layer. We want to do is create the same book for both the Ipad and Kindle environments. Our .PSDs/pages are currently 5413 pixels X 3176 pixels and we have 33 of these files. Our guess is we may have to do some resizing for different platforms and it may not be as easy as simply using a software tool to convert our Nook Book into something submittable to Apple and Amazon? What we need to know is how best to use what we have done so far to create an app for the iPad and Kindle. If all goes well, we hope to create a lot more ebooks/ibooks in the future and are looking to build a long-term relationship.