[VSerge]

Jesse Schell's "the art of game design". It will give you a solid understanding of what game design is and how you can do it. Also, it's a very good read, which doesn't hurt when you're trying to learn something.

I disagree strongly with the advice consisting of: "start hacking and see what happens". For any kind of product design, starting something without any kind of direction is the surest way to go nowhere.