If exporting a book (or sermons, or clippings) to PDF or ePub is too cumbersome to program, perhaps just exporting to HTML. Exporting to HTML would allow for users convert on their own to any number of ebook formats (such as ePub) for use on their e-reader (such as the Nook).
I currently put my sermons on the Nook, then preach from it, rather than note cards.