this book came up on Cool Tools this morning - The Art of Game Design
I actually have that one. It's a pretty cool take on the subject, applying 'lenses' to the game to force decisions about the game and what it will be.
As an example, the first lens is "The Lens of Essential Experience", i.e. stop thinking about the game as a designer and think about the experience of the game as a player.
One of the best quotes in the book is in the beginning - a definition of Game Design as "Deciding what a game should be". There was a companion app on iOS, but not sure if it's still available.
Though he says that he doesn't really agree with books as texts, it really does read like a textbook.