Get Started With Apple’s Swift Programming Language With a Free eBook by Patrick Allan.
Patrick’s blog summarizes a free book on the Swift programming language when he says:
Keep in mind, you will need an iOS device to read the book. (emphasis added)
Can you imagine the hue and cry if Microsoft published a book that required Windows 8 to read it?
I suppose that is one way to keep a market share, make a righteous ghetto out of it.