iOS 4 was released yesterday, and it's already jailbroken. However, iBooks only works in iOS4 on devices jailbroken with PawnageTool 4.01 and redsn0w 0.9.5b5-4 or newer. For some reason I couldn't get PawnageTool to work so I used redsn0w.
The images below are captured from my iPod touch 2G.
The iPhone ebook in the image above is the iPhone SDK Development book in epub format that I bought from The Pragmatic Programmers that I also used in the Amazon Kindle review previously.