The open-source EPUB e-book format is great for sharing books between different readers and making life easier on readers generally--but Amazon's Kindle, the big player, doesn't support it.
The Kobo is an e-reader once sold in Borders bookstores. In addition to reading e-books purchased from the Kobo store, you can read books distributed by your library via the OverDrive system. However, ...
Kindle devices appeal to many people. When everything works, they're great. But Amazon is becoming more and more finicky, both in terms of supported formats and the freedom to manage your own books.