The Swift Programming Language manual available on iBookstore
One of the bigger surprises during today's WWDC 2014 keynote was the announcement of Swift, a new programming language for developing iOS and OS X apps. Apple wasted no time in making the Swift Programming Language manual available online on the iBookstore for free. The 860 page manual contains many code samples and it's suggested that you try them out in Xcode -- in fact, the chapters of the books can be opened as playgrounds in Xcode, giving would-be Swift programmers the ability to edit the code listings and see the results immediately.
