Add more GTK books (#6844)

Author: Hofer-Julian

books/free-programming-books-subjects.md

@@ -301,6 +301,8 @@ 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
+* [GUI development with Relm4](https://aaronerhardt.github.io/relm4-book/book/) - Aaron Erhardt
+* [GUI development with Rust and GTK 4](https://gtk-rs.org/gtk4-rs/stable/latest/book/) - Julian Hofer
 * [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)