O’Reilly recently released the book Beautiful Testing, a collection of essays about testing and QA in general. As I mentioned earlier, I wrote an article in that book on (unit) testing XMPP protocols, using Swift as a motivating example. Since the book’s scope may (oddly enough) not always be as interesting for developers in general, I released my article under a Creative Commons Attribution license (thanks to the good folks from O’Reilly for encouraging us to do this), which you can find here (or directly from my Git repository). The original excerpt from the book (including the index, list of biographies, and all the fancy artwork) is also available for download under the same license.
I of course encourage you to buy either the PDF or dead tree version of the book, as all the proceeds of the book go to charity.