Sorry, Windows users, for the moment it's a Mac-only solution. Another consideration: with iOS 4.3 just around the corner, should you even bother?
Apple has apparently disabled the ability to open already purchased books from its iBooks app on devices that have been jailbroken with a hack called greenpois0n.
Rejoice! Jailbreak for iOS 5.1.1 is upon us! Here's how to jailbreak your iDevice right now.