I would put "The Art of Game Design" at the front of the bookshelf of anyone that does game design work: http://www.amazon.com/The-Art-Game-Design-lenses/dp/01236949.... To me, this is the most valuable book I have on my bookshelf. Every time I pick it up, I scarcely read more than a few pages before I'm setting the book down to make a bunch of notes for new ideas I've had. The book is truly outstanding and practical, and will help give you a framework for figuring out what you want our game to be.

I've been developing games independently now for several years. Easily the most difficult part is the design. Merely implementing the game-- if only that was my biggest challenge!