There are other e-book readers, such as SONY, but you need to consider the range of content available to the non-Kindle readers, compared to what is available for Kindle on amazon.
The non-kindle readers will accept more formats which means that you can get books from more sources. I.e. other booksellers, publishers, etc, that kindle may not accept.
I may be wrong but isn't there a downloadable application called Calibre that reformats ebooks.
So you should be able to change to the e.pub or any other format for Kindle or any other reader.
Check it out!