Learn the things you need for a complete game, such as translations and tutorials, and improve the things you've already written to raise their standard to a professional level. This is a practical guide covering every discipline: art, music, writing, and code. In the case of the latter, code examples are included to demonstrate how to implement functionality to make the game shine.
Polished Game Development acts as a comprehensive checklist of everything your game should, and should not, do, in order to be as good as it possibly can. It is more than just a book on theoretical game design principles.