Candy Crush is a hit game on the App Store. This tutorial walks you through the process of building a match-3 game like Candy Crush in Swift. Taking on a fun project like this is a great way to get up to date in Swift. I like the fact that this tutorial demonstrates how to mix-and-match SpriteKit scenes and nodes with traditional UIKit controls and views.
The folks behind objc.io are iOS wizards. Really. 🎓 They've written some of the best analyses of iOS app architecture and coding practices that I've ever read. Whether you're a novice or a pro, it's well worth your time to read their articles. Learn how to do functional programming in Swift and to use Playgrounds for rapid prototyping.
This introduction to iOS and Objective-C development is an excellent place to start iOS App development. Full of detailed examples and thoughtful discussion of both the how and the why, you'll learn some valuable tips and avoid mistakes by reading this book. Though the book was written for iOS 6, most of it is still highly relevant and it's worth a read.