One book added in GUI section (#6425)

books/free-programming-books-subjects.md

@@ -287,6 +287,7 @@ Books that cover a specific programming language can be found in the  [BY PROGRA
 * [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)
 * [Web Design Primer](https://pressbooks.library.ryerson.ca/webdesign/) - Richard Adams, Ahmed Sagarwala
+* [Web Style Guide Online](https://www.webstyleguide.com/wsg3/index.html) - Patrick J. Lynch and Sarah Horton
 
 
 ### Information Retrieval