Your ebooks are not able to be read on your computer without a compatible eReader app.
PC or Mac
You can only use Adobe Digital Editions. This is a free app. You will need to register for a free Adobe account if you use the computer reader.
Mobile Device (Android or Apple iOS)
We recommend the following eReader app (Android or Apple iOS)
Bluefire Reader
Aldiko (at the time f this post, works well on Android)