Game Programming Patterns : If you need any help in making your games a little more efficient, or maybe you’ve been wondering if theres a different way to write your code, then this is the book for you. It covers different design patterns that you can use for all aspects of your game. Not only can you use these patterns for games but you can learn and re purpose them for any software that you write.
Unity3d Tutorials : If you use Unity, and are starting out, then the Unity tutorial page is a wealth of knowledge. Here you can learn about any subject you might want to know about, making 2/3d games, scripting help, graphics, tips on making a better looking game. Its all here.
itch.io : Itch io is a cool little site for any indie developer or just about any person building games for fun. They have a lot of game related material on the site. Games, TONS of game jams, game resources like sprites and sfx, devlogs that you can use to keep track of your project progress and a really cool community for you to get to know. Definitely check this site out.
The Art of Game Design : Delve deep into what makes a game a great game! An amazing book that goes over the different dimensions of game development, from the psychology, art, audio, etc.. A must read for anyone not just game designers