The Art of Game Design: A Book of Lenses A must have to change your perspectives and improve your designs.
Game Design Workshop, Second Edition: A Playcentric Approach to Creating Innovative Games Every game designer should understand the Playcentric process and principles.
The Art of Game Design: A Deck of Lenses These cards are a must have. Breakthrough your design problems with these cards.
Game Mechanics: Advanced Game Design (Voices That Matter) A serious technical look at game design. Must have for successful game designers.