Update free-programming-books.md

free-programming-books.md

 ###Index
 * [Meta-Lists](#meta-lists)
+* [Graphics user interfaces](#graphics-user-interfaces)
 * [Graphics Programming](#graphics-programming)
 * [Language Agnostic](#language-agnostic)
 * [Android](#android)
@@ -96,6 +97,10 @@
 * [OpenGL Insights](http://openglinsights.com) (chapters 22, 23, 28, 33, 39)
 * [ShaderX series](http://tog.acm.org/resources/shaderx/)
 
+###Graphics User Interfaces
+
+* [Programming with gtkmm 3](https://developer.gnome.org/gtkmm-tutorial/stable/)
+ 
 
 ###Language Agnostic