Requiem, a well known DRM cracking app is at it again, and now has cracked the iBooks Store. DRM's were made when Apple made FairPlay, a content protection service. With it came DRM's, a way for Apple to monitor which devices a certain pece of content can go on. A few years ago, Requiem decided to crack those DRM's, and create a way for music from the iTunes Store, to play on virtually any device. This application works on OS 10.7 Lion, and is compatible with iTunes 10.5.3.
Apple and the developer of Requiem, Brahms, have a cat and mouse game going, and Apple will more than likely release an update to patch this problem. If I were you, I wouldn't do this, as the authors, illustrators, and publishers work hard to get these books out to you to enjoy, and stealing their property just isn't right.