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 * [C# Yellow Book](http://www.csharpcourse.com) (intro to programming)
 * [Creating Mobile Apps with Xamarin.Forms C#](https://developer.xamarin.com/guides/xamarin-forms/creating-mobile-apps-xamarin-forms/)
 * [Daily Design Patterns](https://www.exceptionnotfound.net/downloads/dailydesignpattern.pdf) (PDF)
-* [Data Structures and Algorithms with Object-Oriented Design Patterns in C#](http://www.brpreiss.com/books/opus6/)
+* [Data Structures and Algorithms with Object-Oriented Design Patterns in C#](https://web.archive.org/web/20161220072449/http://www.brpreiss.com/books/opus6/) - Bruno Preiss
 * [Dissecting a C# Application](https://damieng.com/blog/2007/11/08/dissecting-a-c-application-inside-sharpdevelop) - Christian Holm, Bernhard Spuida, Mike Kruger
 * [Fundamentals of Computer Programming with C#](http://www.introprogramming.info/english-intro-csharp-book/read-online/) - Svetlin Nakov
 * [High level asynchronous programming with Reactive Extensions](https://github.com/petroemil/Rx.Book) - Emil Petro