add gtk event driven book (#5933)

* add gtk event driven book

* update with correct book name

Co-authored-by: David Ordás <3125580+davorpa@users.noreply.github.com>

Co-authored-by: David Ordás <3125580+davorpa@users.noreply.github.com>

books/free-programming-books-subjects.md

@@ -262,6 +262,7 @@ Books that cover a specific programming language can be found in the  [BY PROGRA
 
 ### Graphical User Interfaces
 
+* [Event-Driven GTK by Example — 2021 Edition](https://mmstick.github.io/gtkrs-tutorials/) - Michael Murphy
 * [Programming with gtkmm 4](https://developer.gnome.org/gtkmm-tutorial/stable/)
 * [Search User Interfaces](http://searchuserinterfaces.com/book/) - Marti A. Hearst
 * [The GLib/GTK+ Development Platform](https://people.gnome.org/~swilmet/glib-gtk-dev-platform.pdf) - Sébastien Wilmet (PDF)