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.

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.

The Sony Reader
The Sony Reader was one of the first major hardware eBook readers and it is now a very mature device with wide support and distribution.

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.

Barnes & Noble Nook

BeBook

Bookeen Cybook Gen3, Cybook Opus

Boox

COOL-ER

Cruz Tablet

eClicto

ECTACO jetBook and ECTACO jetBook Lite

eSlick

Hanlin eReader

iRex Digital Reader 800, 1000

iRiver Story

Kobo eReader

GNU/Linux tablets and PDAs such as Sharp Zaurus and Nokia 770, n800, n810, and n900

Plastic Logic

PocketBook Reader

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, this is the one you should get for testing.

Adobe Digital Editions (Windows, Apple OS X)
This free program is the reference ePub reader with the best support for ePub and we strongly recommend that you install it for testing your ePub books. It is currently still the only reader that fully supports the ePub table of contents.

FB Reader (Windows, Linux, Apple OS X)
Open source reader software, does not yet support the ePub table of contents or bookmarks.

Google Books (web, Android)
Supports ePub and PDF.

Openberg (Firefox plugin)
Open source reader software, does yet not support the ePub table of contents or bookmarks.

MobiPocket (Windows, Blackberry, Symbian, Windows Mobile)
Freeware, converts ePub to PRC on import.

dotReader (Windows, Linux)
Open source, cross-platform ePub reader, early beta. No support for table of contents.

Lexcycle Stanza (Windows, iPhone, iPod Touch)
Desktop (Windows) and iPhone version available. Supports both bookmarks and the ePub  table of contents. The iPhone/iPod Touch version is available directly from the Apple App Store, which must be accessed through iTunes.

Bookworm (any OS with an Internet connection)
Online open source reader, supports the ePub table of contents. Good HTML and CSS display but requires an active Internet connection.

International Digital Publishing Forum
This is main home page for ePub and everything relating to the ePub format.

Open Packaging Format specification
Full specifications of the OPF ePub format.

TeleRead blog
An informative and entertaining blog about eBooks.

Jedisaber ePub tutorial
An online tutorial explaining how to make ePub eBooks manually. Useful if you want to learn about the format and structure of ePubl eBooks.

Feedbooks
Free classics in ePub format.

Snee
Free children's picture books in ePub format

LexCycle eBook Library
Large library of ePub eBooks maintained by LexCycle, the creators of the Stanza ePub reader for Windows and the Apple iPhone and iPod Touch.