The only reference eBook reader for iBooks is the Apple iPad, with the iBooks app installed from the Apple App Store. If you want to distribute your eBooks for iOS devices you must have access to a physical iPad for testing. You will also need access to an iPhone or iPod Touch if you expect your books to be read on these smaller devices. You can install ePub eBooks on these devices with Apple iTunes. Just connect the device to your computer and drag the ePub file onto the iPad entry in the iTunes window.

It's a good idea to have access to the physical hardware readers you are targeting for testing. Some have more limited capabilities than others.

The reference software reader is the free Adobe Digital Editions. Before trying to create ePub eBooks you should download this free reader from Adobe and install it. If you don't do this you won't be able to view your ePub eBooks on your Windows computer after creating them with Help+Manual. ADE generally provides a good idea of how your eBook is going to look on software readers.

The following hardware readers currently support ePub eBooks, either directly or with free additional software. The most notably absent device on this list at the moment is the Amazon Kindle, which still uses its own closed proprietary format. However, it is rumored that ePub support for the Kindle is in the works and due to be released in the near future.

Dedicated hardware ePub and eBook readers
There are now a wide range of hardware ePub and eBook readers on market. See the Comparison of e-readers on Wikipedia for a good list of the currently available devices and their capabilities.

Apple iPad, iPhone and iPod Touch
All these devices support ePub directly with the iBooks® eBook reader integrated in their iOS operating system. Help+Manual ePub eBooks are fully compatible with Apple iBooks®. There are also other eBook readers for iOS available in the Apple iTunes App Store.

Android smartphones and tablets
A wide variety of ePub readers are available for Android.

Other mobile phones and smartphones
Many other mobile devices also support ePub eBooks.

Software readers for ePub eBooks are already available on many platforms. The following list only includes the known software-based readers that support the ePub format at the time of writing – others may already be available.

The reference software reader is Adobe Digital Editions used to be a good choice but it is now no longer supported so well and it has been involved in controversies regarding Adobe's use of user data. For general testing on Windows, Mac OS and Linux we now recommend the Calibre eBook Reader.

Calibre eBook Reader (Windows, Apple Mac OS, Linux)
This free program is now the software ePub reader with the best support for ePub and we strongly recommend that you install it for testing your ePub books.

Other software eBook readers
There is now such a large number of software-based eBook readers for all current systems that it would be impossible to make any recommendations here. You will need to check current reviews for the best readers for the system you want to use. For testing your own eBooks, however, we can warmly recommend Calibre.

Melody Simmons eBook Indie Covers
Excellent site for everything related to eBook covers and cover images.

International Digital Publishing Forum
This is main home page for ePub and everything relating to the ePub format. This is also where you can find full specifications of the open ePub format.

Project Gutenberg
A huge number of free eBooks in all categories, including both classics and new books.

Feedbooks
Free classics in ePub format.

Snee
Free children's picture books in ePub format