The idea underneath is that the code should not only compile, instead it should "validate". Good code has several characteristics: should be concise, self-explanatory, well organized, well documented, well named, well designed and stand the test of time. The main goals behind the curtain are that clarity always wins over performance and a rationale for a choice should always be provided. Some topics discussed here are general and independent from the language even if everything is tied up to Objective-C.
Zen and the Art of the Objective-C Craftsmanship is really a good book on Objective-C guidelines. Cocoa is full of conventions: if you’re an experienced iOS or OSX developer, you’ll already know most of these best practices; but I bet you’ll also learn two or three things you didn’t know (great discussion on blocks and retain cycles on self).
Luca and Alberto started writing this book on November 2013. But now Swift is the future of Mac and iOS development so they decided to release the current version of their book. Great work (via Dave Verwer’s iOS Dev Weekly).