iOS

iOS is the world’s most advanced mobile operating system, continually redefining what people can do with a mobile device. Together, the iOS SDK and Xcode IDE make it easy for developers to create revolutionary mobile apps.
Site: https://developer.apple.com/technologies/ios

Books

Added Swift Essentials - DZone Refcard

Get ahead of the fastest growing programming language. Swift is an open source, type-safe, object-oriented programming language that is used for more than client-side mobile apps. It is designed for developers to have an easier time writing and maintaining accurate code.

 Free book

Added iOS Succinctly

The companion to Objective-C Succinctly, and your practical introduction to developing for iOS.

 Free book

Added iOS 8 Day by Day

A review of all the important part of iOS 8 for developers. Presented in 39 easy to digest chunks, each with a sample project.

 Free book

Added Swift Cheat Sheet

A short guide to using Apple's new programming language, Swift.

 Free book