1. Positioning by Al Trout 2. Lean Startup by Eric Ries
PS - I help startups with marketing so if you need any advice let me know.
Contact details and bio on my website (see my profile)
The Guerilla Marketing series are all good- by Jay Levinson. As is the oldie but goodie 'Permission Marketing' by Seth Godin.
Weird. I've read almost every book listed so far, and I wouldn't recommend any of them for what you're looking to do.
Most, if not all of these books are strategic. You want tactical. Get a book like "Attention! This Book Will Make You Money" and try out some stuff there.
And good luck on the app!
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