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