There is a free iBook from Stanford on iBookstore that supplements the course material for iOS 6 and XCode 4.5 : Editor's Cut webpage
It's not by the same author, and there are differences in the order things are done, but it is very well written, has animated walkthroughs, and does an excellent job of explaining what has changed and why. A lot less confusing than trying figure out the version differences on one's own while learning a new technology. Of course, as an iBook, it is only viewable on an iOS device, and only usefully so on an iPad.