I love how deep objc.io dives. They'll present long essays on topics which normally get glossed over in a sentence or two. Particularly useful are their thoughts on the state of objective c design patterns (quite a volatile topic since the release of iOS).
I absolutely loved the views and controls articles, something so basic but so usefully in-depth. I acquired many of the facts they mention through trial, error, and WWDC sessions over the years... my coworkers would think I was talking out my ass when I'd bring up the kind of internal plumbing it mentions (e.g., subviews over drawRect or image views being ridiculously fast) and it's nice to have as a resource.
