Starting out with a short one, Herding Code episode 186, Paul Betts on Three Cross-Platform Libraries: Splat, ModernHttpClient and Punchclock.
Paul has some nice libraries designed to simplify cross-platform development in C#. Splat lets you share things in cross-platform viewmodels, the biggest one being images. ModernHttpClient brings platform-specific networking libraries to Xamarin to speed up http calls. Punchclock is a library for managing concurrent operations.