I'm not sure if the fact that it's a game changes anything or not as I don't play in that space myself. But just in terms of marketing in general, some of the essentials include:
The 22 Immutable Laws of Marketing - Al Ries & Jack Trout
Positioning - Al Ries & Jack Trout
Repositioning - Jack Trout (with Steve Rivkin)
Differentiate or Die - Jack Trout (with Steve Rivkin)
The Ultimate Marketing Plan - Dan S. Kennedy
The Ultimate Sales Machine - Chet Holmes
All Marketers Are Liars - Seth Godin
The Purple Cow - Seth Godin
