If you're a fan of the highly popular Stanford iTunes U course on iOS development (CS193P), you'll be happy to learn that an official companion text is now available on iBooks.

iPad and iPhone App Development covers the latest release of Xcode and the newest Objective C enhancements. Approximately 300 pages in length, this is an iPad-only (i.e., iBooks Author) text.

The book supplements and updates the iTunes U videos for Xcode 4.5 and iOS 6. It is available in all 50 iBookstore countries.

Best of all, it's free.