A collection of informative articles and blog posts on writing clean, scalable code and designing applications which I found useful. I'll be updating it as I come across more. Although most of these are in Objective-C, the concepts are valid for coding in Swift too.
Lightweight iOS View Controllers through Separate Data Sources. Guided by MVC by Aleksey Chernish
Avoiding Complex View Controllers by Joe Conway
Don't let your UIViewController think for itself by Ian Keen