Sinch - If you’re building an app that requires instant messaging between users or even voice calls, the Sinch API will allow you to integrate IM, SMS, and Voice calls between users with only a few lines of code.
AppCoda - Appcoda has iOS tutorials on everything from table view models to Map API integration. For extremely detailed and free tutorials with downloadable source code, visit AppCoda’s site.
Tutsplus Learn Android SDK - For beginner Android developers, Tutsplus has a series of free tutorials from setting up to your environment to deploying your application. Similar to AppCoda, Tutsplus makes sure you are following along by giving you the ability to write the code step by step. All of the tutorials are downloadable so you can check out the finished product in case you’re having trouble.
The Swift Programming Language Book - Read our Objective C vs Swift article and now want to learn Swift? Check out Apple’s free Swift programming book packed with code samples and instructional chapters.
Ray Wenderlich - Are you a gamer who wants to start making mobile games? Visit Ray Wenderlich’s blog where he has tutorials for game development for Android and iOS. He goes through Android’s Unity game engine and iOS Sprite Kit for Swift. Ray’s tutorials are extremely detailed and as a beginner, his tutorials are perfect for following step by step. Through code blocks, screenshots, and even video he makes game development less intimidating.