Added book on game design with Adventure Game Studio. (#5335)

books/free-programming-books.md

@@ -670,6 +670,7 @@
 * [Essential Image Optimization](https://images.guide) - Addy Osmani
 * [Foundations of Programming](http://openmymind.net/FoundationsOfProgramming.pdf) (PDF)
 * [Game AI Pro](http://www.gameaipro.com) - Steve Rabin
+* [Game Design with AGS](https://ensadi.github.io/AGSBook/) - Dave Ensminger, A. G. Madi
 * [Game Programming Patterns](http://gameprogrammingpatterns.com) - Bob Nystrom
 * [Getting Real](http://gettingreal.37signals.com)
 * [Google Maps API Succinctly](https://www.syncfusion.com/ebooks/google_maps_api_succinctly) - Mark Lewin