Last month Apress published my book on writing localized apps with Xamarin. It's titled "Cross-platform Localization for Native Mobile Apps with Xamarin" and are on Apress's site or on Amazon. It's on sale through the 21st for 37% off (20 + 17) on the Apress site.
It's not a long book, about 120 pages. It provide the basic information you would need to localize your app for other languages and countries. It's written for the Xamarin developer, but the topics apply to other developers.