https://blogs.msdn.microsoft.com/microsoft_press/2016/03/31/free-ebook-creating-mobile-apps-with-xamarin-forms/
Quoting:
"Microsoft Press, in cooperation with Xamarin, is pleased to publish a new ebook by legendary author Charles Petzold: Creating Mobile Apps with Xamarin.Forms: Cross-platform C# programming for iOS, Android, and Windows."
Free PDF here: http://aka.ms/xamebook
