Add link for book: C# Features Succinctly (#9112)

* Adding link for Angular Testing Succinctly

* Updating link for Angular Testing Succinctly

* Adding link for C# Features Succinctly

* Update books/free-programming-books-langs.md

Co-authored-by: Sharan J <50091666+shrn01@users.noreply.github.com>

Co-authored-by: Sharan J <50091666+shrn01@users.noreply.github.com>

books/free-programming-books-langs.md

@@ -473,6 +473,7 @@ That section got so big, we decided to split it into its own file, the [BY SUBJE
 ### <a id="csharp"></a>C\#
 
 * [Architect Modern Web Applications with ASP.NET Core and Azure](https://docs.microsoft.com/en-us/dotnet/architecture/modern-web-apps-azure/) - Steve "ardalis" Smith
+* [C# Features Succinctly](https://www.syncfusion.com/succinctly-free-ebooks/c-sharp-features-succinctly) - Dirk Strauss (HTML)
 * [C# Notes for Professionals](http://goalkicker.com/CSharpBook/) - Compiled from StackOverflow documentation (PDF)
 * [C# Programming](https://en.wikibooks.org/wiki/C_Sharp_Programming) - Wikibooks
 * [C# Programming Yellow Book](https://www.robmiles.com/s/CSharp-Book-2019-Refresh.pdf) - Rob Miles (PDF) (2019)
@@ -1165,6 +1166,7 @@ That section got so big, we decided to split it into its own file, the [BY SUBJE
 
 * [Angular 1 Style Guide](https://github.com/johnpapa/angular-styleguide/blob/master/a1/README.md) - John Papa (HTML)
 * [Angular for the jQuery developer](http://www.ng-newsletter.com.s3-website-us-east-1.amazonaws.com/posts/angular-for-the-jquery-developer.html) - ng-newsletter, Ari Lerner, David Wu, Nate Murray (HTML)
+* [Angular Testing Succinctly](https://www.syncfusion.com/succinctly-free-ebooks/angular-testing-succinctly) - Joseph D. Booth (HTML)
 * [AngularJS - Step by Logical Step](http://nicholasjohnson.com/angular-book/) - Nicholas Johnson (HTML)
 * [AngularJS Guide](https://docs.angularjs.org/guide/) (HTML)
 * [AngularJS Material Designing](https://material.angularjs.org/latest/) (HTML)