Elegantly creative interface and table of contents design.
Has some UI flaws - basically my devs + designers crowded around excitedly poking and jabbering at it once I DL'd it - and everyone made the SAME mistakes with the UI. That in no way takes away from how innovative and lovely the app is though. It was more of a case of, 'OMG that's so cool, but it would be even cooler if it responded like this to that, etc etc'. It simply shows that it was obviously designed more by artists than by UX folk - which arguably accounts for its brilliance in the first place.
It is a little pricey at US$13.99, but hey... if you'd buy the book anyway, buy the app instead. =)