Godot Engine - Free and open source 2D and 3D game engine
https://godotengine.org/
Godot provides a huge set of common tools, so you can just focus on making your game without reinventing the wheel. Godot is completely free and open-source under the very permissive MIT license. No strings attached, no royalties, nothing. Your game is yours, down to the last line of engine code.
Game AI Pro 3
http://www.gameaipro.com/
All book chapters are free to download. The third volume in the Game AI Pro book series (published June 2017).