C# or Java are enterprisey, established languages used by millions of developers. They aren't going away because of Go, Elixir, or Node.js ...

And C# is sexy, it's the best object oriented language out there. So don't worry. C# doesn't need yet another guide for web apps, there are thousands of books and tutorials about the subject already. You don't need to try selling C# to anyone. People who need it already use it.